Node Docs
How to use the Node.js REPL
description: "REPL stands for Read-Evaluate-Print-Loop, and it's a great way to explore the Node.js features in a quick way"
The node command is the one we use to run our Node.js scripts:
If we omit the filename, we use it in REPL mode:
Note: REPL also known as Read Evaluate Print Loop is a programming language environment (basically a console window) that takes single expression as user input and returns the result back to the console after execution.
If you try it now in your terminal, this is what happens:
the command stays in idle mode and waits for us to enter something.
Tip: if you are unsure how to open your terminal, google "How to open terminal on <your-operating-system>".
The REPL is waiting for us to enter some JavaScript code, to be more precise.
Start simple and enter
The first value, test, is the output we told the console to print, then we get undefined which is the return value of running console.log().
We can now enter a new line of JavaScript.
Use the tab to autocomplete
The cool thing about the REPL is that it's interactive.
As you write your code, if you press the tab key the REPL will try to autocomplete what you wrote to match a variable you already defined or a predefined one.
Exploring JavaScript objects
Try entering the name of a JavaScript class, like Number, add a dot and press tab.
The REPL will print all the properties and methods you can access on that class:
Explore global objects
You can inspect the globals you have access to by typing global. and pressing tab:
The _ special variable
If after some code you type _, that is going to print the result of the last operation.
Dot commands
The REPL has some special commands, all starting with a dot .. They are
.help: shows the dot commands help.editor: enables editor mode, to write multiline JavaScript code with ease. Once you are in this mode, enter ctrl-D to run the code you wrote..break: when inputting a multi-line expression, entering the .break command will abort further input. Same as pressing ctrl-C..clear: resets the REPL context to an empty object and clears any multi-line expression currently being input..load: loads a JavaScript file, relative to the current working directory.save: saves all you entered in the REPL session to a file (specify the filename).exit: exits the repl (same as pressing ctrl-C two times)
The REPL knows when you are typing a multi-line statement without the need to invoke .editor.
For example if you start typing an iteration like this:
and you press enter, the REPL will go to a new line that starts with 3 dots, indicating you can now continue to work on that block.
If you type .break at the end of a line, the multiline mode will stop and the statement will not be executed.
Node.js, accept arguments from the command line
description: 'How to accept arguments in a Node.js program passed from the command line'
You can pass any number of arguments when invoking a Node.js application using
Arguments can be standalone or have a key and a value.
For example:
or
This changes how you will retrieve this value in the Node.js code.
The way you retrieve it is using the process object built into Node.js.
It exposes an argv property, which is an array that contains all the command line invocation arguments.
The first element is the full path of the node command.
The second element is the full path of the file being executed.
All the additional arguments are present from the third position going forward.
You can iterate over all the arguments (including the node path and the file path) using a loop:
You can get only the additional arguments by creating a new array that excludes the first 2 params:
If you have one argument without an index name, like this:
you can access it using
In this case:
args[0] is name=joe, and you need to
parse it. The best way to do so is by using the minimist library, which helps dealing with arguments:
Install the required minimist package using npm (lesson about the package manager comes later on).
This time you need to use double dashes before each argument name:
Output to the command line using Node.js
description: 'How to print to the command line console using Node.js, from the basic console.log to more complex scenarios'
Basic output using the console module
Node.js provides a console module which provides tons of very useful ways to interact with the command line.
It is basically the same as the console object you find in the browser.
The most basic and most used method is console.log(), which prints the string you pass to it to the console.
If you pass an object, it will render it as a string.
You can pass multiple variables to console.log, for example:
and Node.js will print both.
We can also format pretty phrases by passing variables and a format specifier.
For example:
%sformat a variable as a string%dformat a variable as a number%iformat a variable as its integer part only%oformat a variable as an object
Example:
Clear the console
console.clear() clears the console (the behavior might depend on the console used)
Counting elements
console.count() is a handy method.
Take this code:
title="Output to the command line using Node.js"
src="https://stackblitz.com/edit/nodejs-dev-0002-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0002-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
What happens is that console.count() will count the number of times a string is printed, and print the count next to it:
You can just count apples and oranges:
Reset counting
The console.countReset() method resets counter used with console.count().
We will use the apples and orange example to demonstrate this.
Notice how the call to console.countReset('orange') resets the value counter to zero.
Print the stack trace
There might be cases where it's useful to print the call stack trace of a function, maybe to answer the question how did you reach that part of the code?
You can do so using console.trace():
This will print the stack trace. This is what's printed if we try this in the Node.js REPL:
Calculate the time spent
You can easily calculate how much time a function takes to run, using time() and timeEnd()
stdout and stderr
As we saw console.log is great for printing messages in the Console. This is what's called the standard output, or stdout.
console.error prints to the stderr stream.
It will not appear in the console, but it will appear in the error log.
Color the output
You can color the output of your text in the console by using escape sequences. An escape sequence is a set of characters that identifies a color.
Example:
You can try that in the Node.js REPL, and it will print hi! in yellow.
However, this is the low-level way to do this. The simplest way to go about coloring the console output is by using a library. Chalk is such a library, and in addition to coloring it also helps with other styling facilities, like making text bold, italic or underlined.
You install it with npm install chalk, then you can use it:
Using chalk.yellow is much more convenient than trying to remember the escape codes, and the code is much more readable.
Check the project link posted above for more usage examples.
Create a progress bar
Progress is an awesome package to create a progress bar in the console. Install it using npm install progress
This snippet creates a 10-step progress bar, and every 100ms one step is completed. When the bar completes we clear the interval:
Accept input from the command line in Node.js
description: 'How to make a Node.js CLI program interactive using the built-in readline Node.js module'
How to make a Node.js CLI program interactive?
Node.js since version 7 provides the readline module to perform exactly this: get input from a readable stream such as the process.stdin stream, which during the execution of a Node.js program is the terminal input, one line at a time.
This piece of code asks the username, and once the text is entered and the user presses enter, we send a greeting.
The question() method shows the first parameter (a question) and waits for the user input. It calls the callback function once enter is pressed.
In this callback function, we close the readline interface.
readline offers several other methods, and I'll let you check them out on the package documentation linked above.
If you need to require a password, it's best not to echo it back, but instead show a * symbol.
The simplest way is to use the readline-sync package which is very similar in terms of the API and handles this out of the box.
A more complete and abstract solution is provided by the Inquirer.js package.
You can install it using npm install inquirer, and then you can replicate the above code like this:
Inquirer.js lets you do many things like asking multiple choices, having radio buttons, confirmations, and more.
It's worth knowing all the alternatives, especially the built-in ones provided by Node.js, but if you plan to take CLI input to the next level, Inquirer.js is an optimal choice.
Expose functionality from a Node.js file using exports
description: 'How to use the module.exports API to expose data to other files in your application, or to other applications as well'
Node.js has a built-in module system.
A Node.js file can import functionality exposed by other Node.js files.
When you want to import something you use
to import the functionality exposed in the library.js file that resides in the current file folder.
In this file, functionality must be exposed before it can be imported by other files.
Any other object or variable defined in the file by default is private and not exposed to the outer world.
This is what the module.exports API offered by the module system allows us to do.
When you assign an object or a function as a new exports property, that is the thing that's being exposed, and as such, it can be imported in other parts of your app, or in other apps as well.
You can do so in 2 ways.
The first is to assign an object to module.exports, which is an object provided out of the box by the module system, and this will make your file export just that object:
The second way is to add the exported object as a property of exports. This way allows you to export multiple objects, functions or data:
or directly
And in the other file, you'll use it by referencing a property of your import:
or
What's the difference between module.exports and exports?
The first exposes the object it points to.
The latter exposes the properties of the object it points to.
An introduction to the npm package manager
description: 'A quick guide to npm, the powerful package manager key to the success of Node.js. In January 2017 over 350000 packages were reported being listed in the npm registry, making it the biggest single language code repository on Earth, and you can be sure there is a package for (almost!) everything.'
Introduction to npm
npm is the standard package manager for Node.js.
In January 2017 over 350000 packages were reported being listed in the npm registry, making it the biggest single language code repository on Earth, and you can be sure there is a package for (almost!) everything.
It started as a way to download and manage dependencies of Node.js packages, but it has since become a tool used also in frontend JavaScript.
There are many things that npm does.
Yarn and pnpm are alternatives to npm cli. You can check them out as well.
Downloads
npm manages downloads of dependencies of your project.
Installing all dependencies
If a project has a package.json file, by running
it will install everything the project needs, in the node_modules folder, creating it if it's not existing already.
Installing a single package
You can also install a specific package by running
Furthermore, since npm 5, this command adds <package-name> to the package.json file dependencies. Before version 5, you needed to add the flag --save.
Often you'll see more flags added to this command:
--save-devinstalls and adds the entry to thepackage.jsonfile devDependencies--no-saveinstalls but does not add the entry to thepackage.jsonfile dependencies--save-optionalinstalls and adds the entry to thepackage.jsonfile optionalDependencies--no-optionalwill prevent optional dependencies from being installed
Shorthands of the flags can also be used:
-S: --save
-D: --save-dev
-O: --save-optional
The difference between devDependencies and dependencies is that the former contains development tools, like a testing library, while the latter is bundled with the app in production.
As for the optionalDependencies the difference is that build failure of the dependency will not cause installation to fail. But it is your program's responsibility to handle the lack of the dependency. Read more about optional dependencies.
Updating packages
Updating is also made easy, by running
npm will check all packages for a newer version that satisfies your versioning constraints.
You can specify a single package to update as well:
Versioning
In addition to plain downloads, npm also manages versioning, so you can specify any specific version of a package, or require a version higher or lower than what you need.
Many times you'll find that a library is only compatible with a major release of another library.
Or a bug in the latest release of a lib, still unfixed, is causing an issue.
Specifying an explicit version of a library also helps to keep everyone on the same exact version of a package, so that the whole team runs the same version until the package.json file is updated.
In all those cases, versioning helps a lot, and npm follows the semantic versioning (semver) standard.
Running Tasks
The package.json file supports a format for specifying command line tasks that can be run by using
For example:
It's very common to use this feature to run Webpack:
So instead of typing those long commands, which are easy to forget or mistype, you can run
Where does npm install the packages?
description: 'How to find out where npm installs the packages'
When you install a package using npm you can perform 2 types of installation:
a local install
a global install
By default, when you type an npm install command, like:
the package is installed in the current file tree, under the node_modules subfolder.
As this happens, npm also adds the lodash entry in the dependencies property of the package.json file present in the current folder.
A global installation is performed using the -g flag:
When this happens, npm won't install the package under the local folder, but instead, it will use a global location.
Where, exactly?
The npm root -g command will tell you where that exact location is on your machine.
On macOS or Linux this location could be /usr/local/lib/node_modules.
On Windows it could be C:\Users\YOU\AppData\Roaming\npm\node_modules
If you use nvm to manage Node.js versions, however, that location would differ.
I, for example, use nvm and my packages location was shown as /Users/joe/.nvm/versions/node/v8.9.0/lib/node_modules.
How to use or execute a package installed using npm
description: 'How to include and use in your code a package installed in your node_modules folder'
When you install a package into your node_modules folder using npm , or also globally, how do you use it in your Node.js code?
Say you install lodash, the popular JavaScript utility library, using
This is going to install the package in the local node_modules folder.
To use it in your code, you just need to import it into your program using require:
What if your package is an executable?
In this case, it will put the executable file under the node_modules/.bin/ folder.
One easy way to demonstrate this is cowsay.
The cowsay package provides a command line program that can be executed to make a cow say something (and other animals as well 🦊).
When you install the package using npm install cowsay, it will install itself and a few dependencies in the node_modules folder:
There is a hidden .bin folder, which contains symbolic links to the cowsay binaries:
How do you execute those?
You can of course type ./node_modules/.bin/cowsay to run it, and it works, but npx, included in the recent versions of npm (since 5.2), is a much better option. You just run:
and npx will find the package location.
The package.json guide
description: 'The package.json file is a key element in lots of app codebases based on the Node.js ecosystem.'
If you work with JavaScript, or you've ever interacted with a JavaScript project, Node.js or a frontend project, you surely met the package.json file.
What's that for? What should you know about it, and what are some of the cool things you can do with it?
The package.json file is kind of a manifest for your project. It can do a lot of things, completely unrelated. It's a central repository of configuration for tools, for example. It's also where npm and yarn store the names and versions for all the installed packages.
The file structure
Here's an example package.json file:
It's empty! There are no fixed requirements of what should be in a package.json file, for an application. The only requirement is that it respects the JSON format, otherwise it cannot be read by programs that try to access its properties programmatically.
If you're building a Node.js package that you want to distribute over npm things change radically, and you must have a set of properties that will help other people use it. We'll see more about this later on.
This is another package.json:
It defines a name property, which tells the name of the app, or package, that's contained in the same folder where this file lives.
Here's a much more complex example, which was extracted from a sample Vue.js application:
there are lots of things going on here:
versionindicates the current versionnamesets the application/package namedescriptionis a brief description of the app/packagemainsets the entry point for the applicationprivateif set totrueprevents the app/package to be accidentally published onnpmscriptsdefines a set of node scripts you can rundependenciessets a list ofnpmpackages installed as dependenciesdevDependenciessets a list ofnpmpackages installed as development dependenciesenginessets which versions of Node.js this package/app works onbrowserslistis used to tell which browsers (and their versions) you want to support
All those properties are used by either npm or other tools that we can use.
Properties breakdown
Most of those properties are only used on https://www.npmjs.com/, others by scripts that interact with your code, like npm or others.
name
Sets the package name.
Example:
The name must be less than 214 characters, must not have spaces, it can only contain lowercase letters, hyphens (-) or underscores (_).
This is because when a package is published on npm, it gets its own URL based on this property.
If you published this package publicly on GitHub, a good value for this property is the GitHub repository name.
author
Lists the package author name
Example:
Can also be used with this format:
contributors
As well as the author, the project can have one or more contributors. This property is an array that lists them.
Example:
Can also be used with this format:
bugs
Links to the package issue tracker, most likely a GitHub issues page
Example:
homepage
Sets the package homepage
Example:
version
Indicates the current version of the package.
Example:
This property follows the semantic versioning (semver) notation for versions, which means the version is always expressed with 3 numbers: x.x.x.
The first number is the major version, the second the minor version and the third is the patch version.
There is a meaning in these numbers: a release that only fixes bugs is a patch release, a release that introduces backward-compatible changes is a minor release, a major release can have breaking changes.
license
Indicates the license of the package.
Example:
keywords
This property contains an array of keywords that associate with what your package does.
Example:
This helps people find your package when navigating similar packages, or when browsing the https://www.npmjs.com/ website.
description
This property contains a brief description of the package
Example:
This is especially useful if you decide to publish your package to npm so that people can find out what the package is about.
repository
This property specifies where this package repository is located.
Example:
Notice the github prefix. There are other popular services baked in:
You can explicitly set the version control system:
You can use different version control systems:
main
Sets the entry point for the package.
When you import this package in an application, that's where the application will search for the module exports.
Example:
private
if set to true prevents the app/package to be accidentally published on npm
Example:
scripts
Defines a set of node scripts you can run
Example:
These scripts are command line applications. You can run them by calling npm run XXXX or yarn XXXX, where XXXX is the command name. Example: npm run dev.
You can use any name you want for a command, and scripts can do literally anything you want.
dependencies
Sets a list of npm packages installed as dependencies.
When you install a package using npm or yarn:
that package is automatically inserted in this list.
Example:
devDependencies
Sets a list of npm packages installed as development dependencies.
They differ from dependencies because they are meant to be installed only on a development machine, not needed to run the code in production.
When you install a package using npm or yarn:
that package is automatically inserted in this list.
Example:
engines
Sets which versions of Node.js and other commands this package/app work on
Example:
browserslist
Is used to tell which browsers (and their versions) you want to support. It's referenced by Babel, Autoprefixer, and other tools, to only add the polyfills and fallbacks needed to the browsers you target.
Example:
This configuration means you want to support the last 2 major versions of all browsers with at least 1% of usage (from the CanIUse.com stats), except IE8 and lower.
(see more)
Command-specific properties
The package.json file can also host command-specific configuration, for example for Babel, ESLint, and more.
Each has a specific property, like eslintConfig, babel and others. Those are command-specific, and you can find how to use those in the respective command/project documentation.
Package versions
You have seen in the description above version numbers like these: ~3.0.0 or ^0.13.0. What do they mean, and which other version specifiers can you use?
That symbol specifies which updates your package accepts, from that dependency.
Given that using semver (semantic versioning) all versions have 3 digits, the first being the major release, the second the minor release and the third is the patch release, you have these "Rules".
You can combine most of the versions in ranges, like this: 1.0.0 || >=1.1.0 <1.2.0, to either use 1.0.0 or one release from 1.1.0 up, but lower than 1.2.0.
The package-lock.json file
description: "The package-lock.json file is automatically generated when installing node packages. Learn what it's about"
In version 5, npm introduced the package-lock.json file.
What's that? You probably know about the package.json file, which is much more common and has been around for much longer.
The goal of package-lock.json file is to keep track of the exact version of every package that is installed so that a product is 100% reproducible in the same way even if packages are updated by their maintainers.
This solves a very specific problem that package.json left unsolved. In package.json you can set which versions you want to upgrade to (patch or minor), using the semver notation, for example:
if you write
~0.13.0, you want to only update patch releases:0.13.1is ok, but0.14.0is not.if you write
^0.13.0, you want to get updates that do not change the leftmost non-zero number:0.13.1,0.13.2and so on. If you write^1.13.0, you will get patch and minor releases:1.13.1,1.14.0and so on up to2.0.0but not2.0.0.if you write
0.13.0, that is the exact version that will be used, always
You don't commit to Git your node_modules folder, which is generally huge, and when you try to replicate the project on another machine by using the npm install command, if you specified the ~ syntax and a patch release of a package has been released, that one is going to be installed. Same for ^ and minor releases.
If you specify exact versions, like
0.13.0in the example, you are not affected by this problem.
It could be you, or another person trying to initialize the project on the other side of the world by running npm install.
So your original project and the newly initialized project are actually different. Even if a patch or minor release should not introduce breaking changes, we all know bugs can (and so, they will) slide in.
The package-lock.json sets your currently installed version of each package in stone, and npm will use those exact versions when running npm ci.
This concept is not new, and other programming languages package managers (like Composer in PHP) use a similar system for years.
The package-lock.json file needs to be committed to your Git repository, so it can be fetched by other people, if the project is public or you have collaborators, or if you use Git as a source for deployments.
The dependencies versions will be updated in the package-lock.json file when you run npm update.
An example
This is an example structure of a package-lock.json file we get when we run npm install cowsay in an empty folder:
We installed cowsay, which depends on
get-stdinoptimiststring-widthstrip-eof
In turn, those packages require other packages, as we can see from the requires property that some have:
ansi-regexis-fullwidth-code-pointminimistwordwrapstrip-eof
They are added in alphabetical order into the file, and each one has a version field, a resolved field that points to the package location, and an integrity string that we can use to verify the package.
Computer Networking & Socket Programming
Socket programming is the development of programs for Computer Networking. Networking applications are loosely based off of the top most layers of the conceptual 7-layer model called the OSI Model. The layers most relevant to socket programmers is layer 4, the Transport layer, and layer 7, the Application layer. Specific protocols are implemented at each layer, and are instrumental for the functionality of the internet.
Node.js exposes extensive API's for implementing these protocols so developers can create socket programming applications. The most common example is the http module. HTTP exists on the Application layer of the OSI Model; if you want to learn more about the http module, read the Node.js http documentation or the Nodejs.dev guide. Another socket programming Node.js module is the dgram module for UDP communication. UDP exists on the Transport layer; for more information about this technology read the Node.js dgram documentaion. More importantly, the net module, which this guide is all about, provides an extensive API for a stream-based TCP server. TCP exists on the Transport layer of the OSI Model and is what HTTP is implemented on top of.
TCP is a reliable, ordered, and error-checked delivery of a stream of bytes between applications communicating via an internet protocol (IP) network.
~ Wikipedia
Net Module - Getting Started
The key part of the net module is the net.Socket class. A socket instance in Node.js is both a duplex stream and an event emitter. This means it is readable, writable, and can utilize the event loop. Sockets can be created by the user and by Node.js, and they are the programmatic object for communicating. Sockets must connect to a net.Server class instance; a socket connection to a server is often referred to as a client connection. Every socket connection is considered "private" in the sense that the only entities with access to the connection is the server and the client. A server instance can have multiple client connections, but will maintain separate communication streams with each connection.
Lets look at the simplest example of a TCP server, an echo server. An echo server is one which returns the same message sent to it.
With this server running, connect to it using your system's command-line TCP interface. The two most common are telnet and netcat. If you're unsure which, try both! If neither work, search on Google for "telnet alternative for <your OS>".
In another terminal connect to the running TCP server:
If you see the message Client connected! in the TCP server terminal, you can begin sending messages from the client connection terminal. Whatever you send should be sent back to you immediatly (if you copy-and-pasted the code from above directly, you'll receive your message twice because their are two echo implementations). Disconnecting is easy; use your terminal's quit keystroke (commonly CTRL+C) or exit the terminal application instance.
Great work! You have succesfully implemented your first TCP server-client connection.
Example using net.createConnection()
net.createConnection()Before starting, open two terminals and startup the Node.js repl (you can also use two files called server.js and client.js respectively). In the following example, we will create a TCP server and a connection to that server using the net module. By doing so, the interactivity is not the same as the previous echo example. In fact, without using other core Node.js modules, this example is completely non-interactive. The client code will disconnect itself at the end of its execution.
The second example introduces an additional event listener 'end'. This is very useful for determining when a socket connection is terminated. In the client code, the call to client.end() is the programatic way of ending a socket connection.
Conclusion
Fantastic work! With these two examples you should have what you need to get started building your own TCP communication networks. This guide only brushes the surface of socket programming applications, but the most important take away here is that Node.js implements TCP socket connections as duplex streams and event emitters. Understanding those two concepts will benefit your overall understanding of the net module.
For more capabilities of the net module read the Node.js net documentation.
To learn more, try completing the following challenges:
Using
readlineorstreammodules, create an interactive client connection scriptthis can build on the second example provided in this guide
Create a group-chat TCP server
hint: you can hold multiple client connections in an array
use multiple arrays for a multi-channel chat server
for an extra challenge try to implement a basic user-store for authentication of user accounts
Create a mini CRUD database server using an idiomatic chat-command interface
The chat-command interface could use keywords such as
create,read,update, anddeleteso the client can send instructions to the serverData can be persisted between sessions using
fsto read and write data to files
Find the installed version of an npm package
description: 'How to find out which version of a particular package you have installed in your app'
To see the version of all installed npm packages, including their dependencies:
For example:
You can also just open the package-lock.json file, but this involves some visual scanning.
npm list -g is the same, but for globally installed packages.
To get only your top-level packages (basically, the ones you told npm to install and you listed in the package.json), run npm list --depth=0:
You can get the version of a specific package by specifying its name:
This also works for dependencies of packages you installed:
If you want to see what's the latest available version of the package on the npm repository, run npm view [package_name] version:
Install an older version of an npm package
description: 'Learn how to install an older version of an npm package, something that might be useful to solve a compatibility problem'
You can install an old version of an npm package using the @ syntax:
Example:
installs version 1.3.1 (at the time of writing).
Install version 1.2.0 with:
The same can be done with global packages:
You might also be interested in listing all the previous versions of a package. You can do it with npm view <package> versions:
Update all the Node.js dependencies to their latest version
description: 'How do you update all the npm dependencies store in the package.json file, to their latest version available?'
When you install a package using npm install <packagename>, the latest available version of the package is downloaded and put in the node_modules folder, and a corresponding entry is added to the package.json and package-lock.json files that are present in your current folder.
npm calculates the dependencies and installs the latest available version of those as well.
Let's say you install cowsay, a cool command line tool that lets you make a cow say things.
When you npm install cowsay, this entry is added to the package.json file:
and this is an extract of package-lock.json, where we removed the nested dependencies for clarity:
Now those 2 files tell us that we installed version 1.3.1 of cowsay, and our rule for updates is ^1.3.1, which for the npm versioning rules means that npm can update to patch and minor releases: 1.3.2, 1.4.0 and so on.
If there is a new minor or patch release and we type npm update, the installed version is updated, and the package-lock.json file diligently filled with the new version.
Since npm version 5.0.0, npm update will update the package.json with the updated version. Use npm update --no-save to not update package.json.
To discover new releases of the packages, you run npm outdated.
Here's the list of a few outdated packages in one repository that wasn't updated for quite a while:
Some of those updates are major releases. Running npm update won't update the version of those. Major releases are never updated in this way because they (by definition) introduce breaking changes, and npm wants to save you trouble.
To update all packages to a new major version, install the npm-check-updates package globally:
then run it:
this will upgrade all the version hints in the package.json file, to dependencies and devDependencies, so npm can install the new major version.
You are now ready to run the update:
If you just downloaded the project without the node_modules dependencies and you want to install the shiny new versions first, just run
Semantic Versioning using npm
description: 'Semantic Versioning is a convention used to provide a meaning to versions'
If there's one great thing in Node.js packages, it's that they all agreed on using Semantic Versioning for their version numbering.
The Semantic Versioning concept is simple: all versions have 3 digits: x.y.z.
the first digit is the major version
the second digit is the minor version
the third digit is the patch version
When you make a new release, you don't just up a number as you please, but you have rules:
you up the major version when you make incompatible API changes
you up the minor version when you add functionality in a backward-compatible manner
you up the patch version when you make backward-compatible bug fixes
The convention is adopted all across programming languages, and it is very important that every npm package adheres to it, because the whole system depends on that.
Why is that so important?
Because npm set some rules we can use in the package.json file to choose which versions it can update our packages to, when we run npm update.
The rules use those symbols:
^~>>=<<==-||
Let's see those rules in detail:
^: It will only do updates that do not change the leftmost non-zero number i.e there can be changes in minor version or patch version but not in major version. If you write^13.1.0, when runningnpm update, it can update to13.2.0,13.3.0even13.3.1,13.3.2and so on, but not to14.0.0or above.~: if you write~0.13.0when runningnpm updateit can update to patch releases:0.13.1is ok, but0.14.0is not.>: you accept any version higher than the one you specify>=: you accept any version equal to or higher than the one you specify<=: you accept any version equal or lower to the one you specify<: you accept any version lower than the one you specify=: you accept that exact version-: you accept a range of versions. Example:2.1.0 - 2.6.2||: you combine sets. Example:< 2.1 || > 2.6
You can combine some of those notations, for example use 1.0.0 || >=1.1.0 <1.2.0 to either use 1.0.0 or one release from 1.1.0 up, but lower than 1.2.0.
There are other rules, too:
no symbol: you accept only that specific version you specify (
1.2.1)latest: you want to use the latest version available
Uninstalling npm packages
description: 'How to uninstall an npm Node.js package, locally or globally'
To uninstall a package you have previously installed locally (using npm install <package-name> in the node_modules folder, run
from the project root folder (the folder that contains the node_modules folder).
Using the -S flag, or --save, this operation will also remove the reference in the package.json file.
package.json will be automatically updated with devDependency and dependency once you uninstall npm package.
If the package is installed globally, you need to add the -g / --global flag:
for example:
and you can run this command from anywhere you want on your system because the folder where you currently are does not matter.
npm global or local packages
description: 'When is a package best installed globally? Why?'
The main difference between local and global packages is this:
local packages are installed in the directory where you run
npm install <package-name>, and they are put in thenode_modulesfolder under this directoryglobal packages are all put in a single place in your system (exactly where depends on your setup), regardless of where you run
npm install -g <package-name>
In your code you can only require local packages:
so when should you install in one way or another?
In general, all packages should be installed locally.
This makes sure you can have dozens of applications in your computer, all running a different version of each package if needed.
Updating a global package would make all your projects use the new release, and as you can imagine this might cause nightmares in terms of maintenance, as some packages might break compatibility with further dependencies, and so on.
All projects have their own local version of a package, even if this might appear like a waste of resources, it's minimal compared to the possible negative consequences.
A package should be installed globally when it provides an executable command that you run from the shell (CLI), and it's reused across projects.
You can also install executable commands locally and run them using npx, but some packages are just better installed globally.
Great examples of popular global packages which you might know are
npmcreate-react-appvue-cligrunt-climochareact-native-cligatsby-cliforevernodemon
You probably have some packages installed globally already on your system. You can see them by running
on your command line.
npm dependencies and devDependencies
description: 'When is a package a dependency, and when is it a dev dependency?'
When you install an npm package using npm install <package-name>, you are installing it as a dependency.
The package is automatically listed in the package.json file, under the dependencies list (as of npm 5: before you had to manually specify --save).
When you add the -D flag, or --save-dev, you are installing it as a development dependency, which adds it to the devDependencies list.
Development dependencies are intended as development-only packages, that are unneeded in production. For example testing packages, webpack or Babel.
When you go in production, if you type npm install and the folder contains a package.json file, they are installed, as npm assumes this is a development deploy.
You need to set the --production flag (npm install --production) to avoid installing those development dependencies.
The npx Node.js Package Runner
description: 'npx is a very cool way to run Node.js code, and provides many useful features'
npx is a very powerful command that's been available in npm starting version 5.2, released in July 2017.
If you don't want to install npm, you can install npx as a standalone package
npx lets you run code built with Node.js and published through the npm registry.
Easily run local commands
Node.js developers used to publish most of the executable commands as global packages, in order for them to be in the path and executable immediately.
This was a pain because you could not really install different versions of the same command.
Running npx commandname automatically finds the correct reference of the command inside the node_modules folder of a project, without needing to know the exact path, and without requiring the package to be installed globally and in the user's path.
Installation-less command execution
There is another great feature of npx, which is allowing to run commands without first installing them.
This is pretty useful, mostly because:
you don't need to install anything
you can run different versions of the same command, using the syntax @version
A typical demonstration of using npx is through the cowsay command. cowsay will print a cow saying what you wrote in the command. For example:
cowsay "Hello" will print
This only works if you have the cowsay command globally installed from npm previously. Otherwise you'll get an error when you try to run the command.
npx allows you to run that npm command without installing it first. If the command isn't found, npx will install it into a central cache:
will do the job.
Now, this is a funny useless command.
Other scenarios include:
running the
vueCLI tool to create new applications and run them:npx @vue/cli create my-vue-appcreating a new React app using
create-react-app:npx create-react-app my-react-app
and many more.
Run some code using a different Node.js version
Use the @ to specify the version, and combine that with the node npm package:
This helps to avoid tools like nvm or the other Node.js version management tools.
Run arbitrary code snippets directly from a URL
npx does not limit you to the packages published on the npm registry.
You can run code that sits in a GitHub gist, for example:
Of course, you need to be careful when running code that you do not control, as with great power comes great responsibility.
'The Node.js Event Loop'
description: 'The Event Loop is one of the most important aspects to understand about Node.js'
Introduction
The Event Loop is one of the most important aspects to understand about Node.js.
Why is this so important? Because it explains how Node.js can be asynchronous and have non-blocking I/O, and so it explains basically the "killer app" of Node.js, the thing that made it this successful.
The Node.js JavaScript code runs on a single thread. There is just one thing happening at a time.
This is a limitation that's actually very helpful, as it simplifies a lot how you program without worrying about concurrency issues.
You just need to pay attention to how you write your code and avoid anything that could block the thread, like synchronous network calls or infinite loops.
In general, in most browsers there is an event loop for every browser tab, to make every process isolated and avoid a web page with infinite loops or heavy processing to block your entire browser.
The environment manages multiple concurrent event loops, to handle API calls for example. Web Workers run in their own event loop as well.
You mainly need to be concerned that your code will run on a single event loop, and write code with this thing in mind to avoid blocking it.
Blocking the event loop
Any JavaScript code that takes too long to return back control to the event loop will block the execution of any JavaScript code in the page, even block the UI thread, and the user cannot click around, scroll the page, and so on.
Almost all the I/O primitives in JavaScript are non-blocking. Network requests, filesystem operations, and so on. Being blocking is the exception, and this is why JavaScript is based so much on callbacks, and more recently on promises and async/await.
The call stack
The call stack is a LIFO (Last In, First Out) stack.
The event loop continuously checks the call stack to see if there's any function that needs to run.
While doing so, it adds any function call it finds to the call stack and executes each one in order.
You know the error stack trace you might be familiar with, in the debugger or in the browser console? The browser looks up the function names in the call stack to inform you which function originates the current call:
A simple event loop explanation
Let's pick an example:
title="A simple event loop explanation"
src="https://stackblitz.com/edit/nodejs-dev-0003-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0003-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
When this code runs, first foo() is called. Inside foo() we first call bar(), then we call baz().
At this point the call stack looks like this:
The event loop on every iteration looks if there's something in the call stack, and executes it:
until the call stack is empty.
Queuing function execution
The above example looks normal, there's nothing special about it: JavaScript finds things to execute, runs them in order.
Let's see how to defer a function until the stack is clear.
The use case of setTimeout(() => {}, 0) is to call a function, but execute it once every other function in the code has executed.
Take this example:
title="Queuing function execution"
src="https://stackblitz.com/edit/nodejs-dev-0004-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0004-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
This code prints, maybe surprisingly:
When this code runs, first foo() is called. Inside foo() we first call setTimeout, passing bar as an argument, and we instruct it to run immediately as fast as it can, passing 0 as the timer. Then we call baz().
At this point the call stack looks like this:
Here is the execution order for all the functions in our program:
Why is this happening?
The Message Queue
When setTimeout() is called, the Browser or Node.js starts the timer. Once the timer expires, in this case immediately as we put 0 as the timeout, the callback function is put in the Message Queue.
The Message Queue is also where user-initiated events like click or keyboard events, or fetch responses are queued before your code has the opportunity to react to them. Or also DOM events like onload.
The loop gives priority to the call stack, and it first processes everything it finds in the call stack, and once there's nothing in there, it goes to pick up things in the message queue.
We don't have to wait for functions like setTimeout, fetch or other things to do their own work, because they are provided by the browser, and they live on their own threads. For example, if you set the setTimeout timeout to 2 seconds, you don't have to wait 2 seconds - the wait happens elsewhere.
ES6 Job Queue
ECMAScript 2015 introduced the concept of the Job Queue, which is used by Promises (also introduced in ES6/ES2015). It's a way to execute the result of an async function as soon as possible, rather than being put at the end of the call stack.
Promises that resolve before the current function ends will be executed right after the current function.
I find nice the analogy of a rollercoaster ride at an amusement park: the message queue puts you at the back of the queue, behind all the other people, where you will have to wait for your turn, while the job queue is the fastpass ticket that lets you take another ride right after you finished the previous one.
Example:
title="ECMAScript 2015 Job Queue"
src="https://stackblitz.com/edit/nodejs-dev-0005-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0005-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
That's a big difference between Promises (and Async/await, which is built on promises) and plain old asynchronous functions through setTimeout() or other platform APIs.
Finally, here's what the call stack looks like for the example above:
'Understanding process.nextTick()'
description: 'The Node.js process.nextTick function interacts with the event loop in a special way'
As you try to understand the Node.js event loop, one important part of it is process.nextTick().
Every time the event loop takes a full trip, we call it a tick.
When we pass a function to process.nextTick(), we instruct the engine to invoke this function at the end of the current operation, before the next event loop tick starts:
The event loop is busy processing the current function code.
When this operation ends, the JS engine runs all the functions passed to nextTick calls during that operation.
It's the way we can tell the JS engine to process a function asynchronously (after the current function), but as soon as possible, not queue it.
Calling setTimeout(() => {}, 0) will execute the function at the end of next tick, much later than when using nextTick() which prioritizes the call and executes it just before the beginning of the next tick.
Use nextTick() when you want to make sure that in the next event loop iteration that code is already executed.
Introduction to Node.js
description: "Getting started guide to Node.js, the server-side JavaScript runtime environment. Node.js is built on top of the Google Chrome V8 JavaScript engine, and it's mainly used to create web servers - but it's not limited to just that."
Node.js is an open-source and cross-platform JavaScript runtime environment. It is a popular tool for almost any kind of project!
Node.js runs the V8 JavaScript engine, the core of Google Chrome, outside of the browser. This allows Node.js to be very performant.
A Node.js app runs in a single process, without creating a new thread for every request. Node.js provides a set of asynchronous I/O primitives in its standard library that prevent JavaScript code from blocking and generally, libraries in Node.js are written using non-blocking paradigms, making blocking behavior the exception rather than the norm.
When Node.js performs an I/O operation, like reading from the network, accessing a database or the filesystem, instead of blocking the thread and wasting CPU cycles waiting, Node.js will resume the operations when the response comes back.
This allows Node.js to handle thousands of concurrent connections with a single server without introducing the burden of managing thread concurrency, which could be a significant source of bugs.
Node.js has a unique advantage because millions of frontend developers that write JavaScript for the browser are now able to write the server-side code in addition to the client-side code without the need to learn a completely different language.
In Node.js the new ECMAScript standards can be used without problems, as you don't have to wait for all your users to update their browsers - you are in charge of deciding which ECMAScript version to use by changing the Node.js version, and you can also enable specific experimental features by running Node.js with flags.
A Vast Number of Libraries
npm with its simple structure helped the ecosystem of Node.js proliferate, and now the npm registry hosts over 1,000,000 open source packages you can freely use.
An Example Node.js Application
The most common example Hello World of Node.js is a web server:
title="Hello world web server"
src="https://stackblitz.com/edit/nodejs-dev-0001-01?embed=1&file=index.js&zenmode=1"
alt="nodejs-dev-0001-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
This code first includes the Node.js http module.
Node.js has a fantastic standard library, including first-class support for networking.
The createServer() method of http creates a new HTTP server and returns it.
The server is set to listen on the specified port and host name. When the server is ready, the callback function is called, in this case informing us that the server is running.
Whenever a new request is received, the request event is called, providing two objects: a request (an http.IncomingMessage object) and a response (an http.ServerResponse object).
Those 2 objects are essential to handle the HTTP call.
The first provides the request details. In this simple example, this is not used, but you could access the request headers and request data.
The second is used to return data to the caller.
In this case with:
we set the statusCode property to 200, to indicate a successful response.
We set the Content-Type header:
and we close the response, adding the content as an argument to end():
Node.js Frameworks and Tools
Node.js is a low-level platform. In order to make things easy and exciting for developers, thousands of libraries were built upon Node.js by the community.
Many of those established over time as popular options. Here is a non-comprehensive list of the ones worth learning:
AdonisJS: A TypeScript-based fully featured framework highly focused on developer ergonomics, stability, and confidence. Adonis is one of the fastest Node.js web frameworks.
Egg.js: A framework to build better enterprise frameworks and apps with Node.js & Koa.
Express: It provides one of the most simple yet powerful ways to create a web server. Its minimalist approach, unopinionated, focused on the core features of a server, is key to its success.
Fastify: A web framework highly focused on providing the best developer experience with the least overhead and a powerful plugin architecture. Fastify is one of the fastest Node.js web frameworks.
FeatherJS: Feathers is a lightweight web-framework for creating real-time applications and REST APIs using JavaScript or TypeScript. Build prototypes in minutes and production-ready apps in days.
Gatsby: A React-based, GraphQL powered, static site generator with a very rich ecosystem of plugins and starters.
hapi: A rich framework for building applications and services that enables developers to focus on writing reusable application logic instead of spending time building infrastructure.
koa: It is built by the same team behind Express, aims to be even simpler and smaller, building on top of years of knowledge. The new project born out of the need to create incompatible changes without disrupting the existing community.
Loopback.io: Makes it easy to build modern applications that require complex integrations.
Meteor: An incredibly powerful full-stack framework, powering you with an isomorphic approach to building apps with JavaScript, sharing code on the client and the server. Once an off-the-shelf tool that provided everything, now integrates with frontend libs React, Vue, and Angular. Can be used to create mobile apps as well.
Micro: It provides a very lightweight server to create asynchronous HTTP microservices.
NestJS: A TypeScript based progressive Node.js framework for building enterprise-grade efficient, reliable and scalable server-side applications.
Next.js: React framework that gives you the best developer experience with all the features you need for production: hybrid static & server rendering, TypeScript support, smart bundling, route pre-fetching, and more.
Nx: A toolkit for full-stack monorepo development using NestJS, Express, React, Angular, and more! Nx helps scale your development from one team building one application to many teams collaborating on multiple applications!
Sapper: Sapper is a framework for building web applications of all sizes, with a beautiful development experience and flexible filesystem-based routing. Offers SSR and more!
Socket.io: A real-time communication engine to build network applications.
Strapi: Strapi is a flexible, open-source Headless CMS that gives developers the freedom to choose their favorite tools and frameworks while also allowing editors to easily manage and distribute their content. By making the admin panel and API extensible through a plugin system, Strapi enables the world's largest companies to accelerate content delivery while building beautiful digital experiences.
'Understanding setImmediate()'
description: 'The Node.js setImmediate function interacts with the event loop in a special way'
When you want to execute some piece of code asynchronously, but as soon as possible, one option is to use the setImmediate() function provided by Node.js:
Any function passed as the setImmediate() argument is a callback that's executed in the next iteration of the event loop.
How is setImmediate() different from setTimeout(() => {}, 0) (passing a 0ms timeout), and from process.nextTick()?
A function passed to process.nextTick() is going to be executed on the current iteration of the event loop, after the current operation ends. This means it will always execute before setTimeout and setImmediate.
A setTimeout() callback with a 0ms delay is very similar to setImmediate(). The execution order will depend on various factors, but they will be both run in the next iteration of the event loop.
Discover JavaScript Timers
description: 'When writing JavaScript code, you might want to delay the execution of a function. Learn how to use setTimeout and setInterval to schedule functions in the future'
setTimeout()
setTimeout()When writing JavaScript code, you might want to delay the execution of a function.
This is the job of setTimeout. You specify a callback function to execute later, and a value expressing how later you want it to run, in milliseconds:
This syntax defines a new function. You can call whatever other function you want in there, or you can pass an existing function name, and a set of parameters:
setTimeout returns the timer id. This is generally not used, but you can store this id, and clear it if you want to delete this scheduled function execution:
Zero delay
If you specify the timeout delay to 0, the callback function will be executed as soon as possible, but after the current function execution:
This code will print
This is especially useful to avoid blocking the CPU on intensive tasks and let other functions be executed while performing a heavy calculation, by queuing functions in the scheduler.
Some browsers (IE and Edge) implement a
setImmediate()method that does this same exact functionality, but it's not standard and unavailable on other browsers. But it's a standard function in Node.js.
setInterval()
setInterval()setInterval is a function similar to setTimeout, with a difference: instead of running the callback function once, it will run it forever, at the specific time interval you specify (in milliseconds):
The function above runs every 2 seconds unless you tell it to stop, using clearInterval, passing it the interval id that setInterval returned:
It's common to call clearInterval inside the setInterval callback function, to let it auto-determine if it should run again or stop. For example this code runs something unless App.somethingIWait has the value arrived:
Recursive setTimeout
setInterval starts a function every n milliseconds, without any consideration about when a function finished its execution.
If a function always takes the same amount of time, it's all fine:
Maybe the function takes different execution times, depending on network conditions for example:
And maybe one long execution overlaps the next one:
To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes:
to achieve this scenario:
setTimeout and setInterval are available in Node.js, through the Timers module.
Node.js also provides setImmediate(), which is equivalent to using setTimeout(() => {}, 0), mostly used to work with the Node.js Event Loop.
'JavaScript Asynchronous Programming and Callbacks'
description: 'JavaScript is synchronous by default, and is single threaded. This means that code cannot create new threads and run in parallel. Find out what asynchronous code means and how it looks like'
Asynchronicity in Programming Languages
Computers are asynchronous by design.
Asynchronous means that things can happen independently of the main program flow.
In the current consumer computers, every program runs for a specific time slot and then it stops its execution to let another program continue their execution. This thing runs in a cycle so fast that it's impossible to notice. We think our computers run many programs simultaneously, but this is an illusion (except on multiprocessor machines).
Programs internally use interrupts, a signal that's emitted to the processor to gain the attention of the system.
I won't go into the internals of this, but just keep in mind that it's normal for programs to be asynchronous and halt their execution until they need attention, allowing the computer to execute other things in the meantime. When a program is waiting for a response from the network, it cannot halt the processor until the request finishes.
Normally, programming languages are synchronous and some provide a way to manage asynchronicity in the language or through libraries. C, Java, C#, PHP, Go, Ruby, Swift, and Python are all synchronous by default. Some of them handle async by using threads, spawning a new process.
JavaScript
JavaScript is synchronous by default and is single threaded. This means that code cannot create new threads and run in parallel.
Lines of code are executed in series, one after another, for example:
But JavaScript was born inside the browser, its main job, in the beginning, was to respond to user actions, like onClick, onMouseOver, onChange, onSubmit and so on. How could it do this with a synchronous programming model?
The answer was in its environment. The browser provides a way to do it by providing a set of APIs that can handle this kind of functionality.
More recently, Node.js introduced a non-blocking I/O environment to extend this concept to file access, network calls and so on.
Callbacks
You can't know when a user is going to click a button. So, you define an event handler for the click event. This event handler accepts a function, which will be called when the event is triggered:
This is the so-called callback.
A callback is a simple function that's passed as a value to another function, and will only be executed when the event happens. We can do this because JavaScript has first-class functions, which can be assigned to variables and passed around to other functions (called higher-order functions)
It's common to wrap all your client code in a load event listener on the window object, which runs the callback function only when the page is ready:
Callbacks are used everywhere, not just in DOM events.
One common example is by using timers:
XHR requests also accept a callback, in this example by assigning a function to a property that will be called when a particular event occurs (in this case, the state of the request changes):
Handling errors in callbacks
How do you handle errors with callbacks? One very common strategy is to use what Node.js adopted: the first parameter in any callback function is the error object: error-first callbacks
If there is no error, the object is null. If there is an error, it contains some description of the error and other information.
The problem with callbacks
Callbacks are great for simple cases!
However every callback adds a level of nesting, and when you have lots of callbacks, the code starts to be complicated very quickly:
This is just a simple 4-levels code, but I've seen much more levels of nesting and it's not fun.
How do we solve this?
Alternatives to callbacks
Starting with ES6, JavaScript introduced several features that help us with asynchronous code that do not involve using callbacks: Promises (ES6) and Async/Await (ES2017).
Understanding JavaScript Promises
description: 'Promises are one way to deal with asynchronous code in JavaScript, without writing too many callbacks in your code.'
Introduction to promises
title="Introduction to promises"
src="https://stackblitz.com/edit/nodejs-dev-0006-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0006-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
A promise is commonly defined as a proxy for a value that will eventually become available.
Promises are one way to deal with asynchronous code, without getting stuck in callback hell.
Promises have been part of the language for years (standardized and introduced in ES2015), and have recently become more integrated, with async and await in ES2017.
Async functions use promises behind the scenes, so understanding how promises work is fundamental to understanding how async and await work.
How promises work, in brief
Once a promise has been called, it will start in a pending state. This means that the calling function continues executing, while the promise is pending until it resolves, giving the calling function whatever data was being requested.
The created promise will eventually end in a resolved state, or in a rejected state, calling the respective callback functions (passed to then and catch) upon finishing.
Which JS APIs use promises?
In addition to your own code and libraries code, promises are used by standard modern Web APIs such as:
the Battery API
the Fetch API
Service Workers
It's unlikely that in modern JavaScript you'll find yourself not using promises, so let's start diving right into them.
Creating a promise
The Promise API exposes a Promise constructor, which you initialize using new Promise():
As you can see, the promise checks the done global constant, and if that's true, the promise goes to a resolved state (since the resolve callback was called); otherwise, the reject callback is executed, putting the promise in a rejected state. (If none of these functions is called in the execution path, the promise will remain in a pending state)
A more common example you may come across is a technique called Promisifying. This technique is a way to be able to use a classic JavaScript function that takes a callback, and have it return a promise:
In recent versions of Node.js, you won't have to do this manual conversion for a lot of the API. There is a promisifying function available in the util module that will do this for you, given that the function you're promisifying has the correct signature.
Consuming a promise
Now let's see how the promise can be consumed or used.
Running checkIfItsDone() will specify functions to execute when the isItDoneYet promise resolves (in the then call) or rejects (in the catch call).
Chaining promises
A promise can be returned to another promise, creating a chain of promises.
A great example of chaining promises is the Fetch API, which we can use to get a resource and queue a chain of promises to execute when the resource is fetched.
The Fetch API is a promise-based mechanism, and calling fetch() is equivalent to defining our own promise using new Promise().
Example of chaining promises
node-fetchis minimal code for window.fetch compatible API on Node.js runtime.
In this example, we call fetch() to get a list of TODO items from the todos.json file found in the domain root, and we create a chain of promises.
Running fetch() returns a response, which has many properties, and within those we reference:
status, a numeric value representing the HTTP status codestatusText, a status message, which isOKif the request succeeded
response also has a json() method, which returns a promise that will resolve with the content of the body processed and transformed into JSON.
So given those promises, this is what happens: the first promise in the chain is a function that we defined, called status(), that checks the response status and if it's not a success response (between 200 and 299), it rejects the promise.
This operation will cause the promise chain to skip all the chained promises listed and will skip directly to the catch() statement at the bottom, logging the Request failed text along with the error message.
If that succeeds instead, it calls the json() function we defined. Since the previous promise, when successful, returned the response object, we get it as an input to the second promise.
In this case, we return the data JSON processed, so the third promise receives the JSON directly:
and we simply log it to the console.
Handling errors
When anything in the chain of promises fails and raises an error or rejects the promise, the control goes to the nearest catch() statement down the chain.
Cascading errors
If inside the catch() you raise an error, you can append a second catch() to handle it, and so on.
Orchestrating promises
Promise.all()
Promise.all()If you need to synchronize different promises, Promise.all() helps you define a list of promises, and execute something when they are all resolved.
Example:
The ES2015 destructuring assignment syntax allows you to also do
You are not limited to using fetch of course, any promise can be used in this fashion.
Promise.race()
Promise.race()Promise.race() runs when the first of the promises you pass to it settles (resolves or rejects), and it runs the attached callback just once, with the result of the first promise settled.
Example:
Promise.any()
Promise.any()Promise.any() settles when any of the promises you pass to it fulfill or all of the promises get rejected. It returns a single promise that resolves with the value from the first promise that is fulfilled. If all promises are rejected, then the returned promise is rejected with an AggregateError.
Example:
Common errors
Uncaught TypeError: undefined is not a promise
If you get the Uncaught TypeError: undefined is not a promise error in the console, make sure you use new Promise() instead of just Promise()
UnhandledPromiseRejectionWarning
This means that a promise you called rejected, but there was no catch used to handle the error. Add a catch after the offending then to handle this properly.
'Modern Asynchronous JavaScript with Async and Await'
description: 'Discover the modern approach to asynchronous functions in JavaScript. JavaScript evolved in a very short time from callbacks to Promises, and since ES2017 asynchronous JavaScript is even simpler with the async/await syntax'
Introduction
JavaScript evolved in a very short time from callbacks to promises (ES2015), and since ES2017 asynchronous JavaScript is even simpler with the async/await syntax.
Async functions are a combination of promises and generators, and basically, they are a higher level abstraction over promises. Let me repeat: async/await is built on promises.
Why were async/await introduced?
They reduce the boilerplate around promises, and the "don't break the chain" limitation of chaining promises.
When Promises were introduced in ES2015, they were meant to solve a problem with asynchronous code, and they did, but over the 2 years that separated ES2015 and ES2017, it was clear that promises could not be the ultimate solution.
Promises were introduced to solve the famous callback hell problem, but they introduced complexity on their own, and syntax complexity.
They were good primitives around which a better syntax could be exposed to the developers, so when the time was right we got async functions.
They make the code look like it's synchronous, but it's asynchronous and non-blocking behind the scenes.
How it works
An async function returns a promise, like in this example:
When you want to call this function you prepend await, and the calling code will stop until the promise is resolved or rejected. One caveat: the client function must be defined as async. Here's an example:
A quick example
This is a simple example of async/await used to run a function asynchronously:
title="Modern Asynchronous JavaScript with Async and Await"
src="https://stackblitz.com/edit/nodejs-dev-0007-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0007-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
Promise all the things
Prepending the async keyword to any function means that the function will return a promise.
Even if it's not doing so explicitly, it will internally make it return a promise.
This is why this code is valid:
and it's the same as:
The code is much simpler to read
As you can see in the example above, our code looks very simple. Compare it to code using plain promises, with chaining and callback functions.
And this is a very simple example, the major benefits will arise when the code is much more complex.
For example here's how you would get a JSON resource, and parse it, using promises:
And here is the same functionality provided using await/async:
Multiple async functions in series
Async functions can be chained very easily, and the syntax is much more readable than with plain promises:
title="Multiple async functions in series"
src="https://stackblitz.com/edit/nodejs-dev-0008-01?index.js&zenmode=1&view=editor"
alt="nodejs-dev-0008-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
Easier debugging
Debugging promises is hard because the debugger will not step over asynchronous code.
Async/await makes this very easy because to the compiler it's just like synchronous code.
The Node.js Event emitter
description: 'How to work with custom events in Node.js'
If you worked with JavaScript in the browser, you know how much of the interaction of the user is handled through events: mouse clicks, keyboard button presses, reacting to mouse movements, and so on.
On the backend side, Node.js offers us the option to build a similar system using the events module.
This module, in particular, offers the EventEmitter class, which we'll use to handle our events.
You initialize that using
This object exposes, among many others, the on and emit methods.
emitis used to trigger an eventonis used to add a callback function that's going to be executed when the event is triggered
For example, let's create a start event, and as a matter of providing a sample, we react to that by just logging to the console:
When we run
the event handler function is triggered, and we get the console log.
You can pass arguments to the event handler by passing them as additional arguments to emit():
Multiple arguments:
The EventEmitter object also exposes several other methods to interact with events, like
once(): add a one-time listenerremoveListener()/off(): remove an event listener from an eventremoveAllListeners(): remove all listeners for an event
You can read all their details on the events module page at https://nodejs.org/api/events.html
'Build an HTTP Server'
description: 'How to build an HTTP server with Node.js'
Here is a sample Hello World HTTP web server:
title="Build an HTTP Server"
src="https://stackblitz.com/edit/nodejs-dev-0009-01?index.js&zenmode=1"
alt="nodejs-dev-0009-01 on StackBlitz"
style="height: 400px; width: 100%; border: 0;">
Let's analyze it briefly. We include the http module.
We use the module to create an HTTP server.
The server is set to listen on the specified port, 3000. When the server is ready, the listen callback function is called.
The callback function we pass is the one that's going to be executed upon every request that comes in. Whenever a new request is received, the request event is called, providing two objects: a request (an http.IncomingMessage object) and a response (an http.ServerResponse object).
request provides the request details. Through it, we access the request headers and request data.
response is used to populate the data we're going to return to the client.
In this case with
we set the statusCode property to 200, to indicate a successful response.
We also set the Content-Type header:
and we end close the response, adding the content as an argument to end():
'Making HTTP requests with Node.js'
description: 'How to perform HTTP requests with Node.js using GET, POST, PUT and DELETE'
Perform a GET Request
Perform a POST Request
PUT and DELETE
PUT and DELETE requests use the same POST request format - you just need to change the options.method value to the appropriate method.
Make an HTTP POST request using Node.js
description: 'Find out how to make an HTTP POST request using Node.js'
There are many ways to perform an HTTP POST request in Node.js, depending on the abstraction level you want to use.
The simplest way to perform an HTTP request using Node.js is to use the Axios library:
Axios requires the use of a 3rd party library.
A POST request is possible just using the Node.js standard modules, although it's more verbose than the two preceding options:
Get HTTP request body data using Node.js
description: 'Find out how to extract the data sent as JSON through an HTTP request body using Node.js'
Here is how you can extract the data that was sent as JSON in the request body.
If you are using Express, that's quite simple: use the body-parser Node.js module.
For example, to get the body of this request:
This is the matching server-side code:
If you're not using Express and you want to do this in vanilla Node.js, you need to do a bit more work, of course, as Express abstracts a lot of this for you.
The key thing to understand is that when you initialize the HTTP server using http.createServer(), the callback is called when the server got all the HTTP headers, but not the request body.
The request object passed in the connection callback is a stream.
So, we must listen for the body content to be processed, and it's processed in chunks.
We first get the data by listening to the stream data events, and when the data ends, the stream end event is called, once:
So to access the data, assuming we expect to receive a string, we must concatenate the chunks into a string when listening to the stream data, and when the stream end, we parse the string to JSON:
Starting from Node.js v10 a for await .. of syntax is available for use. It simplifies the example above and makes it look more linear:
How to install Node.js
description: 'How you can install Node.js on your system: a package manager, the official website installer or nvm'
Node.js can be installed in different ways. This post highlights the most common and convenient ones.
Official packages for all the major platforms are available at https://nodejs.dev/download/.
One very convenient way to install Node.js is through a package manager. In this case, every operating system has its own.
Other package managers for MacOS, Linux, and Windows are listed in https://nodejs.dev/download/package-manager/
nvm is a popular way to run Node.js. It allows you to easily switch the Node.js version, and install new versions to try and easily rollback if something breaks, for example.
It is also very useful to test your code with old Node.js versions.
See https://github.com/nvm-sh/nvm for more information about this option.
In any case, when Node.js is installed you'll have access to the node executable program in the command line.
'Working with file descriptors in Node.js'
description: 'How to interact with file descriptors using Node.js'
Before you're able to interact with a file that sits in your filesystem, you must get a file descriptor.
A file descriptor is a reference to an open file, a number (fd) returned by opening the file using the open() method offered by the fs module. This number (fd) uniquely identifies an open file in operating system:
Notice the r we used as the second parameter to the fs.open() call.
That flag means we open the file for reading.
Other flags you'll commonly use are
r+open the file for reading and writing, if file don't exist it won't be created.w+open the file for reading and writing, positioning the stream at the beginning of the file. The file is created if not existingaopen the file for writing, positioning the stream at the end of the file. The file is created if not existinga+open the file for reading and writing, positioning the stream at the end of the file. The file is created if not existing
You can also open the file by using the fs.openSync method, which returns the file descriptor, instead of providing it in a callback:
Once you get the file descriptor, in whatever way you choose, you can perform all the operations that require it, like calling fs.close() and many other operations that interact with the filesystem.
'Node.js file stats'
description: 'How to get the details of a file using Node.js'
Every file comes with a set of details that we can inspect using Node.js.
In particular, using the stat() method provided by the fs module.
You call it passing a file path, and once Node.js gets the file details it will call the callback function you pass, with 2 parameters: an error message, and the file stats:
Node.js provides also a sync method, which blocks the thread until the file stats are ready:
The file information is included in the stats variable. What kind of information can we extract using the stats?
A lot, including:
if the file is a directory or a file, using
stats.isFile()andstats.isDirectory()if the file is a symbolic link using
stats.isSymbolicLink()the file size in bytes using
stats.size.
There are other advanced methods, but the bulk of what you'll use in your day-to-day programming is this.
'Node.js File Paths'
description: 'How to interact with file paths and manipulate them in Node.js'
Every file in the system has a path.
On Linux and macOS, a path might look like:
/users/joe/file.txt
while Windows computers are different, and have a structure such as:
C:\users\joe\file.txt
You need to pay attention when using paths in your applications, as this difference must be taken into account.
You include this module in your files using
and you can start using its methods.
Getting information out of a path
Given a path, you can extract information out of it using those methods:
dirname: get the parent folder of a filebasename: get the filename partextname: get the file extension
Example:
You can get the file name without the extension by specifying a second argument to basename:
Working with paths
You can join two or more parts of a path by using path.join():
You can get the absolute path calculation of a relative path using path.resolve():
In this case Node.js will simply append /joe.txt to the current working directory. If you specify a second parameter folder, resolve will use the first as a base for the second:
If the first parameter starts with a slash, that means it's an absolute path:
path.normalize() is another useful function, that will try and calculate the actual path, when it contains relative specifiers like . or .., or double slashes:
Neither resolve nor normalize will check if the path exists. They just calculate a path based on the information they got.
'Reading files with Node.js'
description: 'How to read files using Node.js'
The simplest way to read a file in Node.js is to use the fs.readFile() method, passing it the file path, encoding and a callback function that will be called with the file data (and the error):
Alternatively, you can use the synchronous version fs.readFileSync():
Both fs.readFile() and fs.readFileSync() read the full content of the file in memory before returning the data.
This means that big files are going to have a major impact on your memory consumption and speed of execution of the program.
In this case, a better option is to read the file content using streams.
'Writing files with Node.js'
description: 'How to write files using Node.js'
The easiest way to write to files in Node.js is to use the fs.writeFile() API.
Example:
Alternatively, you can use the synchronous version fs.writeFileSync():
By default, this API will replace the contents of the file if it does already exist.
You can modify the default by specifying a flag:
The flags you'll likely use are
r+open the file for reading and writingw+open the file for reading and writing, positioning the stream at the beginning of the file. The file is created if it does not existaopen the file for writing, positioning the stream at the end of the file. The file is created if it does not exista+open the file for reading and writing, positioning the stream at the end of the file. The file is created if it does not exist
(you can find more flags at https://nodejs.org/api/fs.html#fs_file_system_flags)
Append to a file
A handy method to append content to the end of a file is fs.appendFile() (and its fs.appendFileSync() counterpart):
Using streams
All those methods write the full content to the file before returning the control back to your program (in the async version, this means executing the callback)
In this case, a better option is to write the file content using streams.
'Working with folders in Node.js'
description: 'How to interact with folders using Node.js'
The Node.js fs core module provides many handy methods you can use to work with folders.
Check if a folder exists
Use fs.access() to check if the folder exists and Node.js can access it with its permissions.
Create a new folder
Use fs.mkdir() or fs.mkdirSync() to create a new folder.
Read the content of a directory
Use fs.readdir() or fs.readdirSync() to read the contents of a directory.
This piece of code reads the content of a folder, both files and subfolders, and returns their relative path:
You can get the full path:
You can also filter the results to only return the files, and exclude the folders:
Rename a folder
Use fs.rename() or fs.renameSync() to rename folder. The first parameter is the current path, the second the new path:
fs.renameSync() is the synchronous version:
Remove a folder
Use fs.rmdir() or fs.rmdirSync() to remove a folder.
Removing a folder that has content can be more complicated than you need. You can pass the option { recursive: true } to recursively remove the contents.
NOTE: In Node
v16.xthe optionrecursiveis deprecated forfs.rmdirof callback API, instead usefs.rmto delete folders that have content in them:
Or you can install and make use of the fs-extra module, which is very popular and well maintained. It's a drop-in replacement of the fs module, which provides more features on top of it.
In this case the remove() method is what you want.
Install it using
and use it like this:
It can also be used with promises:
or with async/await:
'The Node.js fs module'
description: 'The fs module of Node.js provides useful functions to interact with the file system'
The fs module provides a lot of very useful functionality to access and interact with the file system.
There is no need to install it. Being part of the Node.js core, it can be used by simply requiring it:
Once you do so, you have access to all its methods, which include:
fs.access(): check if the file exists and Node.js can access it with its permissionsfs.appendFile(): append data to a file. If the file does not exist, it's createdfs.chmod(): change the permissions of a file specified by the filename passed. Related:fs.lchmod(),fs.fchmod()fs.chown(): change the owner and group of a file specified by the filename passed. Related:fs.fchown(),fs.lchown()fs.close(): close a file descriptorfs.copyFile(): copies a filefs.createReadStream(): create a readable file streamfs.createWriteStream(): create a writable file streamfs.link(): create a new hard link to a filefs.mkdir(): create a new folderfs.mkdtemp(): create a temporary directoryfs.open(): set the file modefs.readdir(): read the contents of a directoryfs.readFile(): read the content of a file. Related:fs.read()fs.readlink(): read the value of a symbolic linkfs.realpath(): resolve relative file path pointers (.,..) to the full pathfs.rename(): rename a file or folderfs.rmdir(): remove a folderfs.stat(): returns the status of the file identified by the filename passed. Related:fs.fstat(),fs.lstat()fs.symlink(): create a new symbolic link to a filefs.truncate(): truncate to the specified length the file identified by the filename passed. Related:fs.ftruncate()fs.unlink(): remove a file or a symbolic linkfs.unwatchFile(): stop watching for changes on a filefs.utimes(): change the timestamp of the file identified by the filename passed. Related:fs.futimes()fs.watchFile(): start watching for changes on a file. Related:fs.watch()fs.writeFile(): write data to a file. Related:fs.write()
One peculiar thing about the fs module is that all the methods are asynchronous by default, but they can also work synchronously by appending Sync.
For example:
fs.rename()fs.renameSync()fs.write()fs.writeSync()
This makes a huge difference in your application flow.
Node.js 10 includes experimental support for a promise based API
For example let's examine the fs.rename() method. The asynchronous API is used with a callback:
A synchronous API can be used like this, with a try/catch block to handle errors:
The key difference here is that the execution of your script will block in the second example, until the file operation succeeded.
'The Node.js path module'
description: 'The path module of Node.js provides useful functions to interact with file paths'
The path module provides a lot of very useful functionality to access and interact with the file system.
There is no need to install it. Being part of the Node.js core, it can be used by simply requiring it:
This module provides path.sep which provides the path segment separator (\ on Windows, and / on Linux / macOS), and path.delimiter which provides the path delimiter (; on Windows, and : on Linux / macOS).
These are the path methods:
path.basename()
path.basename()Return the last portion of a path. A second parameter can filter out the file extension:
path.dirname()
path.dirname()Return the directory part of a path:
path.extname()
path.extname()Return the extension part of a path
path.format()
path.format()Returns a path string from an object, This is the opposite of path.parse\
path.format accepts an object as argument with the follwing keys:
root: the rootdir: the folder path starting from the rootbase: the file name + extensionname: the file nameext: the file extension
root is ignored if dir is provided\
ext and name are ignored if base exists
path.isAbsolute()
path.isAbsolute()Returns true if it's an absolute path
path.join()
path.join()Joins two or more parts of a path:
path.normalize()
path.normalize()Tries to calculate the actual path when it contains relative specifiers like . or .., or double slashes:
path.parse()
path.parse()Parses a path to an object with the segments that compose it:
root: the rootdir: the folder path starting from the rootbase: the file name + extensionname: the file nameext: the file extension
Example:
results in
path.relative()
path.relative()Accepts 2 paths as arguments. Returns the relative path from the first path to the second, based on the current working directory.
Example:
path.resolve()
path.resolve()You can get the absolute path calculation of a relative path using path.resolve():
By specifying a second parameter, resolve will use the first as a base for the second:
If the first parameter starts with a slash, that means it's an absolute path:
'The Node.js os module'
description: 'The os module of Node.js provides useful functions to interact with underlying system'
This module provides many functions that you can use to retrieve information from the underlying operating system and the computer the program runs on, and interact with it.
There are a few useful properties that tell us some key things related to handling files:
os.EOL gives the line delimiter sequence. It's on Linux and macOS, and on Windows.
os.constants.signals tells us all the constants related to handling process signals, like SIGHUP, SIGKILL and so on.
os.constants.errno sets the constants for error reporting, like EADDRINUSE, EOVERFLOW and more.
You can read them all on https://nodejs.org/api/os.html#os_signal_constants.
Let's now see the main methods that os provides:
os.arch()
os.arch()Return the string that identifies the underlying architecture, like arm, x64, arm64.
os.cpus()
os.cpus()Return information on the CPUs available on your system.
Example:
os.endianness()
os.endianness()Return BE or LE depending if Node.js was compiled with Big Endian or Little Endian.
os.freemem()
os.freemem()Return the number of bytes that represent the free memory in the system.
os.homedir()
os.homedir()Return the path to the home directory of the current user.
Example:
os.hostname()
os.hostname()Return the host name.
os.loadavg()
os.loadavg()Return the calculation made by the operating system on the load average.
It only returns a meaningful value on Linux and macOS.
Example:
os.networkInterfaces()
os.networkInterfaces()Returns the details of the network interfaces available on your system.
Example:
os.platform()
os.platform()Return the platform that Node.js was compiled for:
darwinfreebsdlinuxopenbsdwin32...more
os.release()
os.release()Returns a string that identifies the operating system release number
os.tmpdir()
os.tmpdir()Returns the path to the assigned temp folder.
os.totalmem()
os.totalmem()Returns the number of bytes that represent the total memory available in the system.
os.type()
os.type()Identifies the operating system:
LinuxDarwinon macOSWindows_NTon Windows
os.uptime()
os.uptime()Returns the number of seconds the computer has been running since it was last rebooted.
os.userInfo()
os.userInfo()Returns an object that contains the current username, uid, gid, shell, and homedir
'The Node.js events module'
description: 'The events module of Node.js provides the EventEmitter class'
The events module provides us the EventEmitter class, which is key to working with events in Node.js.
The event listener has these in-built events:
newListenerwhen a listener is addedremoveListenerwhen a listener is removed
Here's a detailed description of the most useful methods:
emitter.addListener()
emitter.addListener()Alias for emitter.on().
emitter.emit()
emitter.emit()Emits an event. It synchronously calls every event listener in the order they were registered.
emitter.eventNames()
emitter.eventNames()Return an array of strings that represent the events registered on the current EventEmitter object:
emitter.getMaxListeners()
emitter.getMaxListeners()Get the maximum amount of listeners one can add to an EventEmitter object, which defaults to 10 but can be increased or lowered by using setMaxListeners()
emitter.listenerCount()
emitter.listenerCount()Get the count of listeners of the event passed as parameter:
emitter.listeners()
emitter.listeners()Gets an array of listeners of the event passed as parameter:
emitter.off()
emitter.off()Alias for emitter.removeListener() added in Node.js 10
emitter.on()
emitter.on()Adds a callback function that's called when an event is emitted.
Usage:
emitter.once()
emitter.once()Adds a callback function that's called when an event is emitted for the first time after registering this. This callback is only going to be called once, never again.
emitter.prependListener()
emitter.prependListener()When you add a listener using on or addListener, it's added last in the queue of listeners, and called last. Using prependListener it's added, and called, before other listeners.
emitter.prependOnceListener()
emitter.prependOnceListener()When you add a listener using once, it's added last in the queue of listeners, and called last. Using prependOnceListener it's added, and called, before other listeners.
emitter.removeAllListeners()
emitter.removeAllListeners()Removes all listeners of an EventEmitter object listening to a specific event:
emitter.removeListener()
emitter.removeListener()Remove a specific listener. You can do this by saving the callback function to a variable, when added, so you can reference it later:
emitter.setMaxListeners()
emitter.setMaxListeners()Sets the maximum amount of listeners one can add to an EventEmitter object, which defaults to 10 but can be increased or lowered.
How much JavaScript do you need to know to use Node.js?
description: 'If you are just starting out with JavaScript, how deeply do you need to know the language?'
As a beginner, it's hard to get to a point where you are confident enough in your programming abilities.
While learning to code, you might also be confused at where does JavaScript end, and where Node.js begins, and vice versa.
I would recommend you to have a good grasp of the main JavaScript concepts before diving into Node.js:
Lexical Structure
Expressions
Types
Classes
Variables
Functions
this
Arrow Functions
Loops
Scopes
Arrays
Template Literals
Semicolons
Strict Mode
ECMAScript 6, 2016, 2017
With those concepts in mind, you are well on your road to become a proficient JavaScript developer, in both the browser and in Node.js.
The following concepts are also key to understand asynchronous programming, which is one fundamental part of Node.js:
Asynchronous programming and callbacks
Timers
Promises
Async and Await
Closures
The Event Loop
'The Node.js http module'
description: 'The http module of Node.js provides useful functions and classes to build an HTTP server'
The HTTP core module is a key module to Node.js networking.
It can be included using
The module provides some properties and methods, and some classes.
Properties
http.METHODS
http.METHODSThis property lists all the HTTP methods supported:
http.STATUS_CODES
http.STATUS_CODESThis property lists all the HTTP status codes and their description:
http.globalAgent
http.globalAgentPoints to the global instance of the Agent object, which is an instance of the http.Agent class.
It's used to manage connections persistence and reuse for HTTP clients, and it's a key component of Node.js HTTP networking.
More in the http.Agent class description later on.
Methods
http.createServer()
http.createServer()Return a new instance of the http.Server class.
Usage:
http.request()
http.request()Makes an HTTP request to a server, creating an instance of the http.ClientRequest class.
http.get()
http.get()Similar to http.request(), but automatically sets the HTTP method to GET, and calls req.end() automatically.
Classes
The HTTP module provides 5 classes:
http.Agenthttp.ClientRequesthttp.Serverhttp.ServerResponsehttp.IncomingMessage
http.Agent
http.AgentNode.js creates a global instance of the http.Agent class to manage connections persistence and reuse for HTTP clients, a key component of Node.js HTTP networking.
This object makes sure that every request made to a server is queued and a single socket is reused.
It also maintains a pool of sockets. This is key for performance reasons.
http.ClientRequest
http.ClientRequestAn http.ClientRequest object is created when http.request() or http.get() is called.
When a response is received, the response event is called with the response, with an http.IncomingMessage instance as argument.
The returned data of a response can be read in 2 ways:
you can call the
response.read()methodin the
responseevent handler you can setup an event listener for thedataevent, so you can listen for the data streamed into.
http.Server
http.ServerThis class is commonly instantiated and returned when creating a new server using http.createServer().
Once you have a server object, you have access to its methods:
close()stops the server from accepting new connectionslisten()starts the HTTP server and listens for connections
http.ServerResponse
http.ServerResponseCreated by an http.Server and passed as the second parameter to the request event it fires.
Commonly known and used in code as res:
The method you'll always call in the handler is end(), which closes the response, the message is complete and the server can send it to the client. It must be called on each response.
These methods are used to interact with HTTP headers:
getHeaderNames()get the list of the names of the HTTP headers already setgetHeaders()get a copy of the HTTP headers already setsetHeader('headername', value)sets an HTTP header valuegetHeader('headername')gets an HTTP header already setremoveHeader('headername')removes an HTTP header already sethasHeader('headername')return true if the response has that header setheadersSent()return true if the headers have already been sent to the client
After processing the headers you can send them to the client by calling response.writeHead(), which accepts the statusCode as the first parameter, the optional status message, and the headers object.
To send data to the client in the response body, you use write(). It will send buffered data to the HTTP response stream.
If the headers were not sent yet using response.writeHead(), it will send the headers first, with the status code and message that's set in the request, which you can edit by setting the statusCode and statusMessage properties values:
http.IncomingMessage
http.IncomingMessageAn http.IncomingMessage object is created by:
http.Serverwhen listening to therequesteventhttp.ClientRequestwhen listening to theresponseevent
It can be used to access the response:
status using its
statusCodeandstatusMessagemethodsheaders using its
headersmethod orrawHeadersHTTP method using its
methodmethodHTTP version using the
httpVersionmethodURL using the
urlmethodunderlying socket using the
socketmethod
The data is accessed using streams, since http.IncomingMessage implements the Readable Stream interface.
Node.js Buffers
description: 'Learn what Node.js buffers are, what they are used for, how to use them'
What is a buffer?
A buffer is an area of memory. Most JavaScript developers are much less familiar with this concept, compared to programmers using a system programming languages (like C, C++, or Go), which interact directly with memory every day.
It represents a fixed-size chunk of memory (can't be resized) allocated outside of the V8 JavaScript engine.
You can think of a buffer like an array of integers, which each represent a byte of data.
It is implemented by the Node.js Buffer class.
Why do we need a buffer?
Buffers were introduced to help developers deal with binary data, in an ecosystem that traditionally only dealt with strings rather than binaries.
Buffers in Node.js are not related to the concept of buffering data. That is what happens when a stream processor receives data faster than it can digest.
How to create a buffer
A buffer is created using the Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.
Buffer.from(array)Buffer.from(arrayBuffer[, byteOffset[, length]])Buffer.from(buffer)Buffer.from(string[, encoding])
You can also just initialize the buffer passing the size. This creates a 1KB buffer:
While both alloc and allocUnsafe allocate a Buffer of the specified size in bytes, the Buffer created by alloc will be initialized with zeroes and the one created by allocUnsafe will be uninitialized. This means that while allocUnsafe would be quite fast in comparison to alloc, the allocated segment of memory may contain old data which could potentially be sensitive.
Older data, if present in the memory, can be accessed or leaked when the Buffer memory is read. This is what really makes allocUnsafe unsafe and extra care must be taken while using it.
Using a buffer
Access the content of a buffer
A buffer, being an array of bytes, can be accessed like an array:
Those numbers are the UTF-8 bytes that identify the characters in the buffer (H → 72, e → 101, y → 121). This happens because Buffer.from() uses UTF-8 by default.
Keep in mind that some characters may occupy more than one byte in the buffer (é → 195 169).
You can print the full content of the buffer using the toString() method:
buf.toString() also uses UTF-8 by default.
Notice that if you initialize a buffer with a number that sets its size, you'll get access to pre-initialized memory that will contain random data, not an empty buffer!
Get the length of a buffer
Use the length property:
Iterate over the contents of a buffer
Changing the content of a buffer
You can write to a buffer a whole string of data by using the write() method:
Just like you can access a buffer with an array syntax, you can also set the contents of the buffer in the same way:
Slice a buffer
If you want to create a partial visualization of a buffer, you can create a slice. A slice is not a copy: the original buffer is still the source of truth. If that changes, your slice changes.
Use the subarray() method to create it. The first parameter is the starting position, and you can specify an optional second parameter with the end position:
Copy a buffer
Copying a buffer is possible using the set() method:
By default you copy the whole buffer. If you only want to copy a part of the buffer, you can use .subarray() and the offset argument that specifies an offset to write to:
Node.js Streams
description: 'Learn what streams are for, why are they so important, and how to use them.'
What are streams
Streams are one of the fundamental concepts that power Node.js applications.
They are a way to handle reading/writing files, network communications, or any kind of end-to-end information exchange in an efficient way.
Streams are not a concept unique to Node.js. They were introduced in the Unix operating system decades ago, and programs can interact with each other passing streams through the pipe operator (|).
For example, in the traditional way, when you tell the program to read a file, the file is read into memory, from start to finish, and then you process it.
Using streams you read it piece by piece, processing its content without keeping it all in memory.
The Node.js stream module provides the foundation upon which all streaming APIs are built.
All streams are instances of EventEmitter
Why streams
Streams basically provide two major advantages over using other data handling methods:
Memory efficiency: you don't need to load large amounts of data in memory before you are able to process it
Time efficiency: it takes way less time to start processing data, since you can start processing as soon as you have it, rather than waiting till the whole data payload is available
An example of a stream
A typical example is reading files from a disk.
Using the Node.js fs module, you can read a file, and serve it over HTTP when a new connection is established to your HTTP server:
readFile() reads the full contents of the file, and invokes the callback function when it's done.
res.end(data) in the callback will return the file contents to the HTTP client.
If the file is big, the operation will take quite a bit of time. Here is the same thing written using streams:
Instead of waiting until the file is fully read, we start streaming it to the HTTP client as soon as we have a chunk of data ready to be sent.
pipe()
The above example uses the line stream.pipe(res): the pipe() method is called on the file stream.
What does this code do? It takes the source, and pipes it into a destination.
You call it on the source stream, so in this case, the file stream is piped to the HTTP response.
The return value of the pipe() method is the destination stream, which is a very convenient thing that lets us chain multiple pipe() calls, like this:
This construct is the same as doing
Streams-powered Node.js APIs
Due to their advantages, many Node.js core modules provide native stream handling capabilities, most notably:
process.stdinreturns a stream connected to stdinprocess.stdoutreturns a stream connected to stdoutprocess.stderrreturns a stream connected to stderrfs.createReadStream()creates a readable stream to a filefs.createWriteStream()creates a writable stream to a filenet.connect()initiates a stream-based connectionhttp.request()returns an instance of the http.ClientRequest class, which is a writable streamzlib.createGzip()compress data using gzip (a compression algorithm) into a streamzlib.createGunzip()decompress a gzip stream.zlib.createDeflate()compress data using deflate (a compression algorithm) into a streamzlib.createInflate()decompress a deflate stream
Different types of streams
There are four classes of streams:
Readable: a stream you can pipe from, but not pipe into (you can receive data, but not send data to it). When you push data into a readable stream, it is buffered, until a consumer starts to read the data.Writable: a stream you can pipe into, but not pipe from (you can send data, but not receive from it)Duplex: a stream you can both pipe into and pipe from, basically a combination of a Readable and Writable streamTransform: a Transform stream is similar to a Duplex, but the output is a transform of its input
How to create a readable stream
We get the Readable stream from the stream module, and we initialize it and implement the readable._read() method.
First create a stream object:
then implement _read:
You can also implement _read using the read option:
Now that the stream is initialized, we can send data to it:
How to create a writable stream
To create a writable stream we extend the base Writable object, and we implement its _write() method.
First create a stream object:
then implement _write:
You can now pipe a
readable stream in:
How to get data from a readable stream
How do we read data from a readable stream? Using a writable stream:
You can also consume a readable stream directly, using the readable event:
How to send data to a writable stream
Using the stream write() method:
Signaling a writable stream that you ended writing
Use the end() method:
How to create a transform stream
We get the Transform stream from the stream module, and we initialize it and implement the transform._transform() method.
First create a transform stream object:
then implement _transform:
Pipe readable stream:
Node.js, the difference between development and production
description: 'Learn how to set up different configurations for production and development environments'
You can have different configurations for production and development environments.
Node.js assumes it's always running in a development environment.
You can signal Node.js that you are running in production by setting the NODE_ENV=production environment variable.
This is usually done by executing the command
in the shell, but it's better to put it in your shell configuration file (e.g. .bash_profile with the Bash shell) because otherwise the setting does not persist in case of a system restart.
You can also apply the environment variable by prepending it to your application initialization command:
This environment variable is a convention that is widely used in external libraries as well.
Setting the environment to production generally ensures that
logging is kept to a minimum, essential level
more caching levels take place to optimize performance
For example Pug, the templating library used by Express, compiles in debug mode if NODE_ENV is not set to production. Express views are compiled in every request in development mode, while in production they are cached. There are many more examples.
You can use conditional statements to execute code in different environments:
For example, in an Express app, you can use this to set different error handlers per environment:
Error handling in Node.js
description: 'How to handle errors during the execution of a Node.js application'
Errors in Node.js are handled through exceptions.
Creating exceptions
An exception is created using the throw keyword:
As soon as JavaScript executes this line, the normal program flow is halted and the control is held back to the nearest exception handler.
Usually in client-side code value can be any JavaScript value including a string, a number or an object.
In Node.js, we don't throw strings, we just throw Error objects.
Error objects
An error object is an object that is either an instance of the Error object, or extends the Error class, provided in the Error core module:
or
Handling exceptions
An exception handler is a try/catch statement.
Any exception raised in the lines of code included in the try block is handled in the corresponding catch block:
e in this example is the exception value.
You can add multiple handlers, that can catch different kinds of errors.
Catching uncaught exceptions
If an uncaught exception gets thrown during the execution of your program, your program will crash.
To solve this, you listen for the uncaughtException event on the process object:
You don't need to import the process core module for this, as it's automatically injected.
Exceptions with promises
Using promises you can chain different operations, and handle errors at the end:
How do you know where the error occurred? You don't really know, but you can handle errors in each of the functions you call (doSomethingX), and inside the error handler throw a new error, that's going to call the outside catch handler:
To be able to handle errors locally without handling them in the function we call, we can break the chain. You can create a function in each then() and process the exception:
Error handling with async/await
Using async/await, you still need to catch errors, and you do it this way:
How to log an object in Node.js
description: 'Logging objects in Node.js'
When you type console.log() into a JavaScript program that runs in the browser, that is going to create a nice entry in the Browser Console:
Once you click the arrow, the log is expanded, and you can clearly see the object properties:
In Node.js, the same happens.
We don't have such luxury when we log something to the console, because that's going to output the object to the shell if you run the Node.js program manually, or to the log file. You get a string representation of the object.
Now, all is fine until a certain level of nesting. After two levels of nesting, Node.js gives up and prints [Object] as a placeholder:
How can you print the whole object?
The best way to do so, while preserving the pretty print, is to use
where 2 is the number of spaces to use for indentation.
Another option is to use
but the problem is that the nested objects after level 2 are now flattened, and this might be a problem with complex objects.
'Node.js with TypeScript'
description: 'Find out why TypeScript is an awesome tool and learn to use it by yourself.'
What is TypeScript
TypeScript is a very popular open-source language maintained and developed by Microsoft, it's loved and used by a lot of software developers around the world.
Basically, it's a superset of JavaScript that adds new capabilities to the language. Most notable addition are static type definitions, something that is not present in plain JavaScript. Thanks to types, it's possible, for example, to declare what kind of arguments we are expecting and what is returned exactly in our functions or what's the exact shape of the object that we are creating. TypeScript is a really powerful tool and opens new world of possibilities in JavaScript projects. It makes our code more secure and robust by preventing a lot of bugs before code is even shipped - it catches problems during writing the code and integrates wonderfully with code editors like Visual Studio Code.
We can talk about other TypeScript benefits later, let's see some examples now!
Examples
Take a look at this code snippet and then we can unpack it together:
First part with type keyword is responsible for declaring our custom type of objects representing users. Later we utilize this newly created type to create function isAdult that accepts one argument of type User and returns boolean. After this we create justine, our example data that can be used for calling previously defined function. Finally, we create new variable with information whether justine is an adult or not.
There are additional things about this example that you should know. Firstly, if we would not comply with declared types, TypeScript would alarm us that something is wrong and prevent misuse. Secondly, not everything must be typed explicitly - TypeScript is very smart and can deduce types for us. For example, variable isJustineAnAdult would be of type boolean even if we didn't type it explicitly or justine would be valid argument for our function even if we didn't declare this variable as of User type.
Okay, so we have some TypeScript code. Now how do we run it?
First thing to do is to install TypeScript in our project:
Now we can compile it to JavaScript using tsc command in the terminal. Let's do it!
Assuming that our file is named example.ts, the command would look like:
This command will result in a new file named example.js that we can run using Node.js.
Now when we know how to compile and run TypeScript code let's see TypeScript bug-preventing capabilities in action!
This is how we will modify our code:
And this is what TypeScript has to say about this:
As you can see TypeScript successfully prevents us from shipping code that could work unexpectedly. That's wonderful!
More about TypeScript
TypeScript offers a whole lot of other great mechanisms like interfaces, classes, utility types and so on. Also, on bigger projects you can declare your TypeScript compiler configuration in a separate file and granularly adjust how it works, how strict it is and where it stores compiled files for example. You can read more about all this awesome stuff in the official TypeScript docs.
Some of the other benefits of TypeScript that are worth mentioning are that it can be adopted progressively, it helps making code more readable and understandable and it allows developers to use modern language features while shipping code for older Node.js versions.
TypeScript in the Node.js world
TypeScript is well-established in the Node.js world and used by many companies, open-source projects, tools and frameworks.
Some of the notable examples of open-source projects using TypeScript are:
NestJS - robust and fully-featured framework that makes creating scalable and well-architected systems easy and pleasant
TypeORM - great ORM influenced by other well-known tools from other languages like Hibernate, Doctrine or Entity Framework
Prisma - next-generation ORM featuring a declarative data model, generated migrations and fully type-safe database queries
RxJS - widely used library for reactive programming
AdonisJS - A fully featured web framework with Node.js
And many, many more great projects... Maybe even your next one!
Differences between Node.js and the Browser
description: 'How writing JavaScript application in Node.js differs from programming for the Web inside the browser'
Both the browser and Node.js use JavaScript as their programming language.
Building apps that run in the browser is a completely different thing than building a Node.js application.
Despite the fact that it's always JavaScript, there are some key differences that make the experience radically different.
From the perspective of a frontend developer who extensively uses JavaScript, Node.js apps bring with them a huge advantage: the comfort of programming everything - the frontend and the backend - in a single language.
You have a huge opportunity because we know how hard it is to fully, deeply learn a programming language, and by using the same language to perform all your work on the web - both on the client and on the server, you're in a unique position of advantage.
What changes is the ecosystem.
In the browser, most of the time what you are doing is interacting with the DOM, or other Web Platform APIs like Cookies. Those do not exist in Node.js, of course. You don't have the document, window and all the other objects that are provided by the browser.
And in the browser, we don't have all the nice APIs that Node.js provides through its modules, like the filesystem access functionality.
Another big difference is that in Node.js you control the environment. Unless you are building an open source application that anyone can deploy anywhere, you know which version of Node.js you will run the application on. Compared to the browser environment, where you don't get the luxury to choose what browser your visitors will use, this is very convenient.
This means that you can write all the modern ES6-7-8-9 JavaScript that your Node.js version supports.
Since JavaScript moves so fast, but browsers can be a bit slow to upgrade, sometimes on the web you are stuck with using older JavaScript / ECMAScript releases.
You can use Babel to transform your code to be ES5-compatible before shipping it to the browser, but in Node.js, you won't need that.
Another difference is that Node.js uses the CommonJS module system, while in the browser we are starting to see the ES Modules standard being implemented.
In practice, this means that for the time being you use require() in Node.js and import in the browser.
The V8 JavaScript Engine
description: "V8 is the name of the JavaScript engine that powers Google Chrome. It's the thing that takes our JavaScript and executes it while browsing with Chrome. V8 provides the runtime environment in which JavaScript executes. The DOM and the other Web Platform APIs are provided by the browser."
V8 is the name of the JavaScript engine that powers Google Chrome. It's the thing that takes our JavaScript and executes it while browsing with Chrome.
V8 provides the runtime environment in which JavaScript executes. The DOM, and the other Web Platform APIs are provided by the browser.
The cool thing is that the JavaScript engine is independent of the browser in which it's hosted. This key feature enabled the rise of Node.js. V8 was chosen to be the engine that powered Node.js back in 2009, and as the popularity of Node.js exploded, V8 became the engine that now powers an incredible amount of server-side code written in JavaScript.
The Node.js ecosystem is huge and thanks to V8 which also powers desktop apps, with projects like Electron.
Other JS engines
Other browsers have their own JavaScript engine:
Firefox has SpiderMonkey
Safari has JavaScriptCore (also called Nitro)
Edge was originally based on Chakra but has more recently been rebuilt using Chromium and the V8 engine.
and many others exist as well.
All those engines implement the ECMA ES-262 standard, also called ECMAScript, the standard used by JavaScript.
The quest for performance
V8 is written in C++, and it's continuously improved. It is portable and runs on Mac, Windows, Linux and several other systems.
In this V8 introduction, we will ignore the implementation details of V8: they can be found on more authoritative sites (e.g. the V8 official site), and they change over time, often radically.
V8 is always evolving, just like the other JavaScript engines around, to speed up the Web and the Node.js ecosystem.
On the web, there is a race for performance that's been going on for years, and we (as users and developers) benefit a lot from this competition because we get faster and more optimized machines year after year.
Compilation
JavaScript is generally considered an interpreted language, but modern JavaScript engines no longer just interpret JavaScript, they compile it.
This has been happening since 2009, when the SpiderMonkey JavaScript compiler was added to Firefox 3.5, and everyone followed this idea.
JavaScript is internally compiled by V8 with just-in-time (JIT) compilation to speed up the execution.
This might seem counter-intuitive, but since the introduction of Google Maps in 2004, JavaScript has evolved from a language that was generally executing a few dozens of lines of code to complete applications with thousands to hundreds of thousands of lines running in the browser.
Our applications can now run for hours inside a browser, rather than being just a few form validation rules or simple scripts.
In this new world, compiling JavaScript makes perfect sense because while it might take a little bit more to have the JavaScript ready, once done it's going to be much more performant than purely interpreted code.
Run Node.js scripts from the command line
description: 'How to run any Node.js script from the CLI'
The usual way to run a Node.js program is to run the node globally available command (once you install Node.js) and pass the name of the file you want to execute.
If your main Node.js application file is app.js, you can call it by typing:
Above, you are explicitly telling the shell to run your script with node. You can also embed this information into your JavaScript file with a "shebang" line. The "shebang" is the first line in the file, and tells the OS which interpreter to use for running the script. Below is the first line of JavaScript:
Above, we are explicitly giving the absolute path of interpreter. Not all operating systems have node in the bin folder, but all should have env. You can tell the OS to run env with node as parameter:
To use a shebang, your file should have executable permission. You can give app.js the executable permission by running:
While running the command, make sure you are in the same directory which contains the app.js file.
How to exit from a Node.js program
description: 'Learn how to terminate a Node.js app in the best possible way'
There are various ways to terminate a Node.js application.
When running a program in the console you can close it with ctrl-C, but what we want to discuss here is programmatically exiting.
Let's start with the most drastic one, and see why you're better off not using it.
The process core module provides a handy method that allows you to programmatically exit from a Node.js program: process.exit().
When Node.js runs this line, the process is immediately forced to terminate.
This means that any callback that's pending, any network request still being sent, any filesystem access, or processes writing to stdout or stderr - all is going to be ungracefully terminated right away.
If this is fine for you, you can pass an integer that signals the operating system the exit code:
By default the exit code is 0, which means success. Different exit codes have different meaning, which you might want to use in your own system to have the program communicate to other programs.
You can read more on exit codes at https://nodejs.org/api/process.html#process_exit_codes
You can also set the process.exitCode property:
and when the program ends, Node.js will return that exit code.
A program will gracefully exit when all the processing is done.
Many times with Node.js we start servers, like this HTTP server:
Express is a framework that uses the http module under the hood, app.listen() returns an instance of http. You would use https.createServer if you needed to serve your app using HTTPS, as app.listen only uses the http module.
This program is never going to end. If you call process.exit(), any currently pending or running request is going to be aborted. This is not nice.
In this case you need to send the command a SIGTERM signal, and handle that with the process signal handler:
Note:
processdoes not require a "require", it's automatically available.
What are signals? Signals are a POSIX intercommunication system: a notification sent to a process in order to notify it of an event that occurred.
SIGKILL is the signal that tells a process to immediately terminate, and would ideally act like process.exit().
SIGTERM is the signal that tells a process to gracefully terminate. It is the signal that's sent from process managers like upstart or supervisord and many others.
You can send this signal from inside the program, in another function:
Or from another Node.js running program, or any other app running in your system that knows the PID of the process you want to terminate.
How to read environment variables from Node.js
description: 'Learn how to read and make use of environment variables in a Node.js program'
The process core module of Node.js provides the env property which hosts all the environment variables that were set at the moment the process was started.
The below code runs app.js and set USER_ID and USER_KEY.
That will pass the user USER_ID as 239482 and the USER_KEY as foobar. This is suitable for testing, however for production, you will probably be configuring some bash scripts to export variables.
Note:
processdoes not require a "require", it's automatically available.
Here is an example that accesses the USER_ID and USER_KEY environment variables, which we set in above code.
In the same way you can access any custom environment variable you set.
If you have multiple environment variables in your node project, you can also create an .env file in the root directory of your project, and then use the dotenv package to load them during runtime.
In your js file
You can also run your js file with
node -r dotenv/config index.jscommand if you don't want to import the package in your code.
'Node.js with WebAssembly'
description: 'Find out why WebAssembly is an awesome tool and learn to use it by yourself.'
WebAssembly is a high-performance assembly-like language that can be compiled from a myriad of languages including C/C++, Rust, and AssemblyScript. As of right now, it is supported by Chrome, Firefox, Safari, Edge, and Node.js!
The WebAssembly specification details two file formats, a binary format called a WebAssembly Module with a .wasm extension and corresponding text representation called WebAssembly Text format with a .wat extension.
Key Concepts
Module - A compiled WebAssembly binary, ie a
.wasmfile.Memory - A resizable ArrayBuffer.
Table - A resizable typed array of references not stored in Memory.
Instance - An instantiation of a Module with its Memory, Table, and variables.
In order to use WebAssembly, you need a .wasm binary file and a set of APIs to communicate with WebAssembly. Node.js provides the necessary APIs via the global WebAssembly object.
Generating WebAssembly Modules
There are multiple methods available to generate WebAssembly binary files including:
Writing WebAssembly (
.wat) by hand and converting to binary format using tools such as wabtUsing emscripten with a C/C++ application
Using wasm-pack with a Rust application
Using AssemblyScript if you prefer a TypeScript-like experience
Some of these tools generate not only the binary file, but the JavaScript "glue" code and corresponding HTML files to run in the browser.
How to use it
Once you have a WebAssembly module, you can use the Node.js WebAssembly object to instantiate it.
Interacting with the OS
WebAssembly modules cannot directly access OS functionality on its own. A third-party tool Wasmtime can be used to access this functionality. Wasmtime utilizes the WASI API to access the OS functionality.
Resources
General WebAssembly Information
MDN Docs
Write WebAssembly by hand
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