Introducing JSX

Consider this variable declaration:

```const element = <h1>Hello, world! </h1>```

This funny tag syntax is neither a string nor HTML.nn

It is called JSX, and it is a syntax extension to JavaScript. We recommend using it with React to describe what the UI should look like. JSX may remind you of a template langu age, but it comes with the full power of JavaScript.

Why JSX?

React embraces the fact that rendering logic is inherently coupled with other UI logic: how events are handled, how the state changes over time, and how the data is prepared for display.

Instead of artificially separating technologies by putting markup and logic in separate files, React separates concerns with loosely coupled units called "components" that contain both logic and markup

React doesn't require using JSX, but most people find it helpful as a visual aid when working with UI inside the JavaScript code. It also allows React to show more useful error and warning messages.

Embedding Expressions in JSX

Whenever JavaScript constants or expressions are to be written inside JSX, they should be wrapped in curly braces. In the example below, we declared a variable called name and then use it inside JSX by wrapping it in curly braces:

const intro = Welcome to JSX from W3resorce;
const jsxElement = <h1>Hello, {intro}</h1>;

As illustrated in the above code snippet, you can put any valid JavaScript expression inside the curly braces in JSX.

In the example below, we embed the result of calling a JavaScript function, formatName(user), into an <h1> element.</h1>

function formatName(user) {
  return user.firstName + ' ' + user.lastName;

const user = {
  firstName: Ogbonna,
  lastName: Vitalis

const element = (
    Hello, {formatName(user)}!


JSX as an Expression

After compilation, JSX expressions become regular JavaScript function calls and evaluate to JavaScript objects.

This means that you can use JSX inside of if statements and for loops, assign it to variables, accept it as arguments, and return it from functions:

function getGreeting(user) {
  if (user) {
    return <h1>Hello, {formatName(user)}!</h1>;
  return <h1>Hello, Stranger.</h1>;

Specifying Attributes with JSX

Attributes in JSX are specified in quotes.

const element = <div tabIndex="0"></div>;

You may also use curly braces to embed a JavaScript expression in an attribute:

const element = <img src={user.avatarUrl}></img>;

Don't put quotes around curly braces when embedding a JavaScript expression in an attribute. You should either use quotes (for string values) or curly braces (for expressions), but not both in the same attribute.


ISince JSX is closer to JavaScript than to HTML, React DOM uses camelCase property naming convention instead of HTML attribute names.

For example, class becomes className in JSX, and tabindex becomes tabIndex.

Specifying Children with JSX

For every open JXS tag, there must be a corresponding closing JXS tag. If a tag is empty, you may close it immediately with />, like XML. The illustration below will clearify

const element = <img src={user.avatarUrl} />;

JSX tags may also contain other JSX tags as their children, as illustrated in the snippet below.

const element = (
    <h2>Good to see you here.</h2>

Preventing Injection Attacks with JSX

Embedding user input in JSX is completely safe, this is because by default React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that's not explicitly written in your application. Everything is converted to a string before being rendered. This helps prevent XSS (cross-site-scripting) attacks.

const title = response.potentiallyMaliciousInput;
// This is safe:
const element = <h1&{title}</h1>;

JSX Represents Objects

When JSX are transpiled, they compile down to React.createElement() calls.

In React, these two examples are equal and identical

const element = (
  <h1 className="greeting">
    Hello, world!
const element = React.createElement(
  {className: 'greeting'},
  'Hello, world!'

React.createElement() performs a few checks to help you write bug-free code but essentially it creates an object like this:

// Note: this structure is simplified
const element = {
  type: 'h1',
  props: {
    className: 'greeting',
    children: 'Hello, world!'

These objects are called "React elements". You can think of them as descriptions of what you want to see on the screen. React reads these objects and uses them to construct the DOM and keep it up to date

In the next tutorial, we explore the act of rendering react elements on the DOM.

Previous: Components and Props
Next: Hello World Application in React