Introduction to React for Complete Beginners

All of the code examples below will be included a second time at the bottom of this article as an embedded gist.

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Introduction to React for Complete Beginners

All of the code examples below will be included a second time at the bottom of this article as an embedded gist, so that it is properly syntax highlighted.

React uses a syntax extension of JavaScript called JSX that allows you to write HTML directly within JavaScript.

React

React uses a syntax extension of JavaScript called JSX that allows you to write HTML directly within JavaScript

because JSX is a syntactic extension of JavaScript, you can actually write JavaScript directly within JSX

include the code you want to be treated as JavaScript within curly braces: { ‘this is treated as JavaScript code’ }

JSX code must be compiled into JavaScript

under the hood the challenges are calling ReactDOM.render (JSX, document.getElementById(‘root’))

One important thing to know about nested JSX is that it must return a single element.

For instance, several JSX elements written as siblings with no parent wrapper element will not transpile.

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From the React Docs

What is React?

React is a declarative, efficient, and flexible JavaScript library for building user interfaces. It lets you compose complex UIs from small and isolated pieces of code called “components”.

React has a few different kinds of components, but we’ll start with React.Component subclasses:

class ShoppingList extends React.Component {
  render() {
    return (
      <div className="shopping-list">
        <h1>Shopping List for {this.props.name}</h1>
        <ul>
          <li>Instagram</li>
          <li>WhatsApp</li>
          <li>Oculus</li>
        </ul>
      </div>
    );
  }
}

// Example usage: <ShoppingList name="Mark" />

We’ll get to the funny XML-like tags soon. We use components to tell React what we want to see on the screen. When our data changes, React will efficiently update and re-render our components.

Here, ShoppingList is a React component class, or React component type. A component takes in parameters, called props (short for “properties”), and returns a hierarchy of views to display via the render method.

The render method returns a description of what you want to see on the screen. React takes the description and displays the result. In particular, render returns a React element, which is a lightweight description of what to render. Most React developers use a special syntax called “JSX” which makes these structures easier to write. The <div /> syntax is transformed at build time to React.createElement('div'). The example above is equivalent to:

return React.createElement('div', {className: 'shopping-list'},
  React.createElement('h1', /* ... h1 children ... */),
  React.createElement('ul', /* ... ul children ... */)
);

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Valid JSX

<div>
  <p>Paragraph One</p>
  <p>Paragraph Two</p>
  <p>Paragraph Three</p>
</div>

---

Invalid JSX

<p>Paragraph One</p>
<p>Paragraph Two</p>
<p>Paragraph Three</p>

To put comments inside JSX, you use the syntax {/* */} to wrap around the comment text

To put comments inside JSX, you use the syntax {/* */} to wrap around the comment text.

The code editor has a JSX element similar to what you created in the last challenge. Add a comment somewhere within the provided div element, without modifying the existing h1 or p elements.

const JSX = (
  <div>
  {/* This is a comment */}
    <h1>This is a block of JSX</h1>
    <p>Here's a subtitle</p>
  </div>
);

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With React, we can render this JSX directly to the HTML DOM using React’s rendering API known as ReactDOM.

ReactDOM offers a simple method to render React elements to the DOM which looks like this:

ReactDOM.render(componentToRender, targetNode)

• the first argument is the React element or component that you want to render,

• and the second argument is the DOM node that you want to render the component to.

ReactDOM.render() must be called after the JSX element declarations, just like how you must declare variables before using them.

key difference in JSX is that you can no longer use the word class to define HTML classes.

• — -> This is because class is a reserved word in JavaScript. Instead, JSX uses className

the naming convention for all HTML attributes and event references in JSX become camelCase

a click event in JSX is onClick, instead of onclick. Likewise, onchange becomes onChange. While this is a subtle difference, it is an important one to keep in mind moving forward.

Apply a class of myDiv to the div provided in the JSX code

• The constant JSX should return a div element.

• The div should have a class of myDiv.

const JSX = (
  <div>
    <h1>Add a class to this div</h1>
  </div>
);

Ans

const JSX = (
  <div className="myDiv">
    <h1>Add a class to this div</h1>
  </div>
);

React: Learn About Self-Closing JSX Tags

-Another important way in which JSX differs from HTML is in the idea of the self-closing tag.

In HTML, almost all tags have both an opening and closing tag: <div></div>; the closing tag always has a forward slash before the tag name that you are closing.

there are special instances in HTML called “self-closing tags”, or tags that don’t require both an opening and closing tag before another tag can start.

For example the line-break tag can be written as <br> or as <br />, but should never be written as <br></br>, since it doesn't contain any content.

In JSX, the rules are a little different. Any JSX element can be written with a self-closing tag, and every element must be closed. The line-break tag, for example, must always be written as <br /> in order to be valid JSX that can be transpiled. A <div>, on the other hand, can be written as <div />or<div></div>. The difference is that in the first syntax version there is no way to include anything in the <div />.

Fix the errors in the code editor so that it is valid JSX and successfully transpiles. Make sure you don’t change any of the content — you only need to close tags where they are needed

const JSX = (
  <div>
    <h2>Welcome to React!</h2> <br >
    <p>Be sure to close all tags!</p>
    <hr >
  </div>
);

Ans

const JSX = (
  <div>
    <h2>Welcome to React!</h2> <br />
    <p>Be sure to close all tags!</p>
    <hr />
  </div>
);

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React: Create a Stateless Functional Component

There are two ways to create a React component. The first way is to use a JavaScript function.

Defining a component in this way creates a stateless functional component.

think of a stateless component as one that can receive data and render it, but does not manage or track changes to that data.

To create a component with a function, you simply write a JavaScript function that returns either JSX or null

• React requires your function name to begin with a capital letter.

Here’s an example of a stateless functional component that assigns an HTML class in JSX:

// After being transpiled, the <div> will have a CSS class of 'customClass'
const DemoComponent = function() {
  return (
    <div className='customClass' />
  );
};

Because a JSX component represents HTML, you could put several components together to create a more complex HTML page.

The code editor has a function called MyComponent. Complete this function so it returns a single div element which contains some string of text

Note: The text is considered a child of the div element, so you will not be able to use a self-closing tag.

const MyComponent = function() {
  // Change code below this line

  // Change code above this line
}

ANS

const MyComponent = function() {
  // Change code below this line

return (
   <div> Some Text </div >
  );

  // Change code above this line
};

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React: Create a React Component

The other way to define a React component is with the ES6 class syntax. In the following example, Kitten extends React.Component:

class Kitten extends React.Component {
  constructor(props) {
    super(props);
  }

  render() {
    return (
      <h1>Hi</h1>
    );
  }
}

This creates an ES6 class Kitten which extends the React.Component class.

So the Kitten class now has access to many useful React features, such as local state and lifecycle hooks.

Also notice the Kitten class has a constructor defined within it that calls super()

It uses super() to call the constructor of the parent class, in this case React.Component

The constructor is a special method used during the initialization of objects that are created with the class keyword. It is best practice to call a component’s constructor with super, and pass props to both.

This makes sure the component is initialized properly. For now, know that it is standard for this code to be included.

MyComponent is defined in the code editor using class syntax. Finish writing the render method so it returns a div element that contains an h1 with the text Hello React

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    // Change code below this line

    // Change code above this line
  }
};

ANS

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    // Change code below this line
 return (
   <div>
      <h1>Hello React!</h1>
      </div>
    );

    // Change code above this line
  }
};

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React: Create a Component with Composition

Imagine you are building an App and have created three components, a Navbar, Dashboard, and Footer.

To compose these components together, you could create an App parent component which renders each of these three components as children. To render a component as a child in a React component, you include the component name written as a custom HTML tag in the JSX.

• For example, in the render method you could write:

return (
 <App>
  <Navbar />
  <Dashboard />
  <Footer />
 </App>
)

When React encounters a custom HTML tag that references another component (a component name wrapped in < /> like in this example), it renders the markup for that component in the location of the tag. This should illustrate the parent/child relationship between the App component and the Navbar, Dashboard, and Footer.

Challenge

In the code editor, there is a simple functional component called ChildComponent and a class component called ParentComponent. Compose the two together by rendering the ChildComponent within the ParentComponent. Make sure to close the ChildComponent tag with a forward slash.

• Note:ChildComponent is defined with an ES6 arrow function because this is a very common practice when using React.

• However, know that this is just a function.

const ChildComponent = () => {
  return (
    <div>
      <p>I am the child</p>
    </div>
  );
};

class ParentComponent extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    return (
      <div>
        <h1>I am the parent</h1>
        { /* Change code below this line */ }

        { /* Change code above this line */ }
      </div>
    );
  }
};

⌛The React component should return a single div element. ⌛The component should return two nested elements. ⌛The component should return the ChildComponent as its second child.

Ans

const ChildComponent = () => {
  return (
    <div>
      <p>I am the child</p>
    </div>
  );
};

class ParentComponent extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    return (
      <div>
        <h1>I am the parent</h1>
        { /* Change code below this line */ }

        { /* Change code above this line */ }
      </div>
    );
  }
};

More Examples

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bgoonz - Overview Web Developer, Electrical Engineer https://bryanguner.medium.com/ https://portfolio42.netlify.app/…github.com

More content at plainenglish.io

By Bryan Guner on May 19, 2021.

Canonical link

Exported from Medium on August 31, 2021.