Static Type Checking
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Static type checkers like and identify certain types of problems before you even run your code. They can also improve developer workflow by adding features like auto-completion. For this reason, we recommend using Flow or TypeScript instead of PropTypes
for larger code bases.
is a static type checker for your JavaScript code. It is developed at Facebook and is often used with React. It lets you annotate the variables, functions, and React components with a special type syntax, and catch mistakes early. You can read an to learn its basics.
To use Flow, you need to:
Add Flow to your project as a dependency.
Ensure that Flow syntax is stripped from the compiled code.
Add type annotations and run Flow to check them.
We will explain these steps below in detail.
First, navigate to your project directory in the terminal. You will need to run the following command:
If you use , run:
If you use , run:
This command installs the latest version of Flow into your project.
Now, add flow
to the "scripts"
section of your package.json
to be able to use this from the terminal:
Finally, run one of the following commands:
This command will create a Flow configuration file that you will need to commit.
Flow extends the JavaScript language with a special syntax for type annotations. However, browsers aren't aware of this syntax, so we need to make sure it doesn't end up in the compiled JavaScript bundle that is sent to the browser.
The exact way to do this depends on the tools you use to compile JavaScript.
Create React App
Babel
Note:
These instructions are not for Create React App users. Even though Create React App uses Babel under the hood, it is already configured to understand Flow. Only follow this step if you don't use Create React App.
If you manually configured Babel for your project, you will need to install a special preset for Flow.
If you use Yarn, run:
If you use npm, run:
This will let you use the Flow syntax in your code.
Note:
Flow does not require the
react
preset, but they are often used together. Flow itself understands JSX syntax out of the box.
Other Build Setups
If you followed the instructions above, you should be able to run Flow for the first time.
If you use npm, run:
You should see a message like:
By default, Flow only checks the files that include this annotation:
Typically it is placed at the top of a file. Try adding it to some files in your project and run yarn flow
or npm run flow
to see if Flow already found any issues.
Now you're all set! We recommend to check out the following resources to learn more about Flow:
To use TypeScript, you need to:
Add TypeScript as a dependency to your project
Configure the TypeScript compiler options
Use the right file extensions
Add definitions for libraries you use
Let's go over these in detail.
Create React App supports TypeScript out of the box.
To create a new project with TypeScript support, run:
Note:
If you use Create React App, you can skip the rest of this page. It describes the manual setup which doesn't apply to Create React App users.
It all begins with running one command in your terminal.
Congrats! You've installed the latest version of TypeScript into your project. Installing TypeScript gives us access to the tsc
command. Before configuration, let's add tsc
to the "scripts" section in our package.json
:
The compiler is of no help to us until we tell it what to do. In TypeScript, these rules are defined in a special file called tsconfig.json
. To generate this file:
Of the many options, we'll look at rootDir
and outDir
. In its true fashion, the compiler will take in typescript files and generate javascript files. However we don't want to get confused with our source files and the generated output.
We'll address this in two steps:
Firstly, let's arrange our project structure like this. We'll place all our source code in the src
directory.
Next, we'll tell the compiler where our source code is and where the output should go.
Generally, you don't want to keep the generated javascript in your source control, so be sure to add the build folder to your .gitignore
.
In React, you most likely write your components in a .js
file. In TypeScript we have 2 file extensions:
.ts
is the default file extension while .tsx
is a special extension used for files which contain JSX
.
If you followed the instructions above, you should be able to run TypeScript for the first time.
If you use npm, run:
If you see no output, it means that it completed successfully.
To be able to show errors and hints from other packages, the compiler relies on declaration files. A declaration file provides all the type information about a library. This enables us to use javascript libraries like those on npm in our project.
There are two main ways to get declarations for a library:
Bundled - The library bundles its own declaration file. This is great for us, since all we need to do is install the library, and we can use it right away. To check if a library has bundled types, look for an index.d.ts
file in the project. Some libraries will have it specified in their package.json
under the typings
or types
field.
Local Declarations Sometimes the package that you want to use doesn't bundle declarations nor is it available on DefinitelyTyped. In that case, we can have a local declaration file. To do this, create a declarations.d.ts
file in the root of your source directory. A simple declaration could look like this:
You are now ready to code! We recommend to check out the following resources to learn more about TypeScript:
If you use , run:
If you use , run:
If your project was set up using , congratulations! The Flow annotations are already being stripped by default so you don't need to do anything else in this step.
Then add the flow
preset to your . For example, if you configure Babel through .babelrc
file, it could look like this:
If you don't use either Create React App or Babel, you can use to strip the type annotations.
There is also to force Flow to check all files regardless of the annotation. This can be too noisy for existing projects, but is reasonable for a new project if you want to fully type it with Flow.
is a programming language developed by Microsoft. It is a typed superset of JavaScript, and includes its own compiler. Being a typed language, TypeScript can catch errors and bugs at build time, long before your app goes live. You can learn more about using TypeScript with React .
You can also add it to an existing Create React App project, .
If you use , run:
If you use , run:
If you use , run:
If you use , run:
Looking at the now generated tsconfig.json
, you can see that there are many options you can use to configure the compiler. For a detailed description of all the options, check .
Great! Now when we run our build script the compiler will output the generated javascript to the build
folder. The provides a tsconfig.json
with a good set of rules to get you started.
- DefinitelyTyped is a huge repository of declarations for libraries that don't bundle a declaration file. The declarations are crowd-sourced and managed by Microsoft and open source contributors. React for example doesn't bundle its own declaration file. Instead we can get it from DefinitelyTyped. To do so enter this command in your terminal.
is a typed language that compiles to JavaScript. Some of its core features are guaranteed 100% type coverage, first-class JSX support and to allow integration in existing JS / TS React codebases.
You can find more infos on integrating ReScript in your existing JS / React codebase .
is a statically typed language developed by JetBrains. Its target platforms include the JVM, Android, LLVM, and .
JetBrains develops and maintains several tools specifically for the React community: as well as . The latter helps you start building React apps with Kotlin with no build configuration.
Note there are other statically typed languages that compile to JavaScript and are thus React compatible. For example, with . Check out their respective sites for more information, and feel free to add more statically typed languages that work with React to this page!