React and JSX

yhtbiyoxe1asgx4tIntroducing JSX

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”. We will explore rendering them to the DOM in the next section. Below, you can find the basics of JSX necessary to get you started.

Embedding Expressions in JSX

You can embed any JavaScript expression in JSX by wrapping it in curly braces.

For example, 2 + 2, user.name, and formatName(user) are all valid expressions:

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

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

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

ReactDOM.render(
  element,
  document.getElementById('root')
);

Try it on CodePen.

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

JSX is an Expression Too

After compilation, JSX expressions become regular 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

You may use quotes to specify string literals as attributes:

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>;

Specifying Children with JSX

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>
);

Caveat:

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

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

JSX Prevents Injection Attacks

It is safe to embed user input in JSX:

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

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.

JSX Represents Objects

Babel compiles JSX down to React.createElement() calls.

These two examples are identical:

const element = (
  <h1 className="greeting">
    Hello, world!
  </h1>
);
const element = React.createElement(
  'h1',
  {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.

We will explore rendering React elements to the DOM in the next section.

Tip:

We recommend searching for a “Babel” syntax scheme for your editor of choice so that both ES6 and JSX code is properly highlighted.

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React: Rendering Elements

Elements are the smallest building blocks of React apps.

An element describes what you want to see on the screen:

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

Unlike browser DOM elements, React elements are plain objects, and are cheap to create. React DOM takes care of updating the DOM to match the React elements.

Note:

One might confuse elements with a more widely known concept of “components”. We will introduce components in the next section. Elements are what components are “made of”, and we encourage you to read this section before jumping ahead.

Rendering an Element into the DOM

Let’s say there is a

somewhere in your HTML file:

id="root">

We call this a “root” DOM node because everything inside it will be managed by React DOM.

Applications built with just React usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like.

To render a React element into a root DOM node, pass both to ReactDOM.render():

const element = <h1>Hello, world</h1>;
ReactDOM.render(
  element,
  document.getElementById('root')
);

Try it on CodePen.

It displays “Hello World” on the page.

Updating the Rendered Element

React elements are immutable. Once you create an element, you can’t change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.

With our knowledge so far, the only way to update the UI is to create a new element, and pass it to ReactDOM.render().

Consider this ticking clock example:

function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(
    element,
    document.getElementById('root')
  );
}

setInterval(tick, 1000);

Try it on CodePen.

It calls ReactDOM.render() every second from a setInterval() callback.

Note:

In practice, most React apps only call ReactDOM.render() once. In the next sections we will learn how such code gets encapsulated into stateful components.

We recommend that you don’t skip topics because they build on each other.

React Only Updates What’s Necessary

React DOM compares the element and its children to the previous one, and only applies the DOM updates necessary to bring the DOM to the desired state.

You can verify by inspecting the last example with the browser tools:

DOM inspector showing granular updates

Even though we create an element describing the whole UI tree on every tick, only the text node whose contents has changed gets updated by React DOM.

In our experience, thinking about how the UI should look at any given moment rather than how to change it over time eliminates a whole class of bugs.

React: Components and Props

Components let you split the UI into independent, reusable pieces, and think about each piece in isolation.

Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called “props”) and return React elements describing what should appear on the screen.

Functional and Class Components

The simplest way to define a component is to write a JavaScript function:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

This function is a valid React component because it accepts a single “props” object argument with data and returns a React element. We call such components “functional” because they are literally JavaScript functions.

You can also use an ES6 class to define a component:

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

The above two components are equivalent from React’s point of view.

Classes have some additional features that we will discuss in the next sections. Until then, we will use functional components for their conciseness.

Rendering a Component

Previously, we only encountered React elements that represent DOM tags:

const element = <div />;

However, elements can also represent user-defined components:

const element = <Welcome name="Sara" />;

When React sees an element representing a user-defined component, it passes JSX attributes to this component as a single object. We call this object “props”.

For example, this code renders “Hello, Sara” on the page:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

const element = <Welcome name="Sara" />;
ReactDOM.render(
  element,
  document.getElementById('root')
);

Try it on CodePen.

Let’s recap what happens in this example:

  1. We call ReactDOM.render() with the <Welcome name="Sara" /> element.
  2. React calls the Welcome component with {name: 'Sara'} as the props.
  3. Our Welcome component returns a <h1>Hello, Sara</h1> element as the result.
  4. React DOM efficiently updates the DOM to match <h1>Hello, Sara</h1>.

Caveat:

Always start component names with a capital letter.

For example, <div /> represents a DOM tag, but <Welcome /> represents a component and requires Welcome to be in scope.

Composing Components

Components can refer to other components in their output. This lets us use the same component abstraction for any level of detail. A button, a form, a dialog, a screen: in React apps, all those are commonly expressed as components.

For example, we can create an App component that renders Welcome many times:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

function App() {
  return (
    <div>
      <Welcome name="Sara" />
      <Welcome name="Cahal" />
      <Welcome name="Edite" />
    </div>
  );
}

ReactDOM.render(
  <App />,
  document.getElementById('root')
);

Try it on CodePen.

Typically, new React apps have a single App component at the very top. However, if you integrate React into an existing app, you might start bottom-up with a small component like Button and gradually work your way to the top of the view hierarchy.

Caveat:

Components must return a single root element. This is why we added a <div> to contain all the <Welcome /> elements.

Extracting Components

Don’t be afraid to split components into smaller components.

For example, consider this Comment component:

function Comment(props) {
  return (
    <div className="Comment">
      <div className="UserInfo">
        <img className="Avatar"
          src={props.author.avatarUrl}
          alt={props.author.name}
        />
        <div className="UserInfo-name">
          {props.author.name}
        </div>
      </div>
      <div className="Comment-text">
        {props.text}
      </div>
      <div className="Comment-date">
        {formatDate(props.date)}
      </div>
    </div>
  );
}

Try it on CodePen.

It accepts author (an object), text (a string), and date (a date) as props, and describes a comment on a social media website.

This component can be tricky to change because of all the nesting, and it is also hard to reuse individual parts of it. Let’s extract a few components from it.

First, we will extract Avatar:

function Avatar(props) {
  return (
    <img className="Avatar"
      src={props.user.avatarUrl}
      alt={props.user.name}
    />
  );
}

The Avatar doesn’t need to know that it is being rendered inside a Comment. This is why we have given its prop a more generic name: user rather than author.

We recommend naming props from the component’s own point of view rather than the context in which it is being used.

We can now simplify Comment a tiny bit:

function Comment(props) {
  return (
    <div className="Comment">
      <div className="UserInfo">
        <Avatar user={props.author} />
        <div className="UserInfo-name">
          {props.author.name}
        </div>
      </div>
      <div className="Comment-text">
        {props.text}
      </div>
      <div className="Comment-date">
        {formatDate(props.date)}
      </div>
    </div>
  );
}

Next, we will extract a UserInfo component that renders an Avatar next to user’s name:

function UserInfo(props) {
  return (
    <div className="UserInfo">
      <Avatar user={props.user} />
      <div className="UserInfo-name">
        {props.user.name}
      </div>
    </div>
  );
}

This lets us simplify Comment even further:

function Comment(props) {
  return (
    <div className="Comment">
      <UserInfo user={props.author} />
      <div className="Comment-text">
        {props.text}
      </div>
      <div className="Comment-date">
        {formatDate(props.date)}
      </div>
    </div>
  );
}

Try it on CodePen.

Extracting components might seem like grunt work at first, but having a palette of reusable components pays off in larger apps. A good rule of thumb is that if a part of your UI is used several times (Button, Panel, Avatar), or is complex enough on its own (App, FeedStory, Comment), it is a good candidate to be a reusable component.

Props are Read-Only

Whether you declare a component as a function or a class, it must never modify its own props. Consider this sum function:

function sum(a, b) {
  return a + b;
}

Such functions are called “pure” because they do not attempt to change their inputs, and always return the same result for the same inputs.

In contrast, this function is impure because it changes its own input:

function withdraw(account, amount) {
  account.total -= amount;
}

React is pretty flexible but it has a single strict rule:

All React components must act like pure functions with respect to their props.

Of course, application UIs are dynamic and change over time. In the next section, we will introduce a new concept of “state”. State allows React components to change their output over time in response to user actions, network responses, and anything else, without violating this rule.