React JSX Intro

What is JSX?

JSX stands for JavaScript XML.

JSX allows us to write HTML in React.

JSX makes it easier to write and add HTML in React.

Coding JSX

JSX allows us to write HTML elements in JavaScript and place them in the DOM without any createElement() and/or appendChild() methods.

JSX converts HTML tags into react elements.

Note: You are not required to use JSX, but JSX makes it easier to write React applications.

Here are two examples. The first uses JSX and the second does not:

Example 1

const myElement = <h1>I Love JSX!</h1>;

createRoot(document.getElementById('root')).render(
  myElement
);

Example 2

const myElement = React.createElement('h1', {}, 'I do not use JSX!');

createRoot(document.getElementById('root')).render(
  myElement
);

As you can see in the first example, JSX allows us to write HTML directly within the JavaScript code.

JSX is an extension of the JavaScript language based on ES6, and is translated into regular JavaScript at runtime.

Expressions in JSX

With JSX you can write expressions inside curly braces { }.

The expression can be a React variable, or property, or any other valid JavaScript expression. JSX will execute the expression and return the result:

Example

const myElement = <h1>React is {5 + 5} times better with JSX</h1>;

Inserting a Large Block of HTML

To write HTML on multiple lines, put the HTML inside parentheses:

Example

const myElement = (
  <ul>
    <li>Apples</li>
    <li>Bananas</li>
    <li>Cherries</li>
  </ul>
);

One Top Level Element

The HTML code must be wrapped in ONE top level element.

So if you like to write two paragraphs, you must put them inside a parent element, like a div element.

Example

const myElement = (
  <div>
    <p>I am a paragraph.</p>
    <p>I am a paragraph too.</p>
  </div>
);

Note: JSX will throw an error if the HTML is not correct, or if the HTML misses a parent element.

Alternatively, you can use a "fragment" to wrap multiple lines. This will prevent unnecessarily adding extra nodes to the DOM.

A fragment looks like an empty HTML tag: <></>.

Example

const myElement = (
  <>
    <p>I am a paragraph.</p>
    <p>I am a paragraph too.</p>
  </>
);

Elements Must be Closed

JSX follows XML rules, and therefore HTML elements must be properly closed.

Example

const myElement = <input type="text" />;

Note: JSX will throw an error if the HTML is not properly closed.

Attribute class = className

The class attribute is a much used attribute in HTML, but since JSX is rendered as JavaScript, and the class keyword is a reserved word in JavaScript, you are not allowed to use it in JSX.

Note: Use attribute className instead.

JSX solved this by using className instead. When JSX is rendered, it translates className attributes into class attributes.

Example

const myElement = <h1 className="myclass">Hello World</h1>;

Comments in JSX

Comments in JSX are written with {/* */}

Example

const myElement = <h1>Hello {/* Wonderful */} World </h1>;

JSX in React Components

React uses components to build UIs. Components are independent and reusable bits of code.

React components are like JavaScript functions, and return HTML.

JSX works perfect inside React components

Example

function Car() {
  return (
    <>
      <h2>My Car</h2>
      <p>It is a Ford Mustang.</p>
    </>
  );
}

createRoot(document.getElementById('root')).render(
  <Car />
)

Inside the components, you can do some other operations before returning the HTML:

Example

function Car() {
  const brand = "Ford";
  const model = "Mustang";
  return (
    <>
      <h2>My Car</h2>
      <p>It is a {brand} {model}.</p>
    </>
  );
}

createRoot(document.getElementById('root')).render(
  <Car />
)