Introducing JSX

Introducing JSX
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.

Consider this variable declaration:

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

This funny tag syntax is neither a string nor HTML. 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 language, but it comes with the full power of JavaScript.

JSX produces React “elements”. Below, you can find the basics of JSX necessary to get you started.

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. 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
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.

In the example below, we declare a variable called name and then use it inside JSX by wrapping it in curly braces:

const name = 'Josh Perez';
const element = <h1>Hello, {name}</h1>;

You can put any valid JavaScript expression inside the curly braces in JSX. For example, 2 + 2, user.firstName, or formatName(user) are all valid JavaScript expressions.

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

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

const user = {
 firstName: 'Harper',
 lastName: 'Perez'
};

const element = (
 <h1>Hello, {formatName(user)}!</h1>
);

We split JSX over multiple lines for readability. While it isn’t required, when doing this, we also recommend wrapping it in parentheses to avoid the pitfalls of automatic semicolon insertion.

JSX is an Expression Too
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.

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
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.

You may use quotes to specify string literals as attributes:

const element = <a href = "https://www.reactjs.org">link</a>;

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

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

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.

Warning:

Since 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
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.
  • Mrs. Gardiner, to whom the chief of this news had been given before, in the course of Jane and Elizabeth’s correspondence with her, made her sister a slight answer, and, in compassion to her nieces, turned the conversation.

If a tag is empty, you may close it immediately with />, like XML:

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

JSX tags may contain children:

const element = (
 <div>
  <h1>Hello!</h1>
  <h2>Good to see you here.</h2>
 </div>
);