Responding to Events

React lets you add event handlers to your JSX. Event handlers are your own functions that will be triggered in response to user interactions like clicking, hovering, focusing on form inputs, and so on.

You will learn

  • Different ways to write an event handler
  • How to pass event handling logic from a parent component
  • How events propagate and how to stop them

Adding event handlers

To add an event handler, you will first define a function and then pass it as a prop to the appropriate JSX tag. For example, here is a button that doesn’t do anything yet:

export default function Button() {
  return (
    <button>
      I don't do anything
    </button>
  );
}

You can make it show a message when a user clicks by following these three steps:

  1. Declare a function called handleClick inside your Button component.
  2. Implement the logic inside that function (use alert to show the message).
  3. Add onClick={handleClick} to the <button> JSX.
export default function Button() {
  function handleClick() {
    alert('You clicked me!');
  }

  return (
    <button onClick={handleClick}>
      Click me
    </button>
  );
}

You defined the handleClick function and then passed it as a prop to <button>. handleClick is an event handler. Event handler functions:

  • Are usually defined inside your components.
  • Have names that start with handle, followed by the name of the event.

While there is no special syntax for event handlers, it is a convention to name them handle followed by the event handled. You’ll often see onClick={handleClick}, onMouseEnter={handleMouseEnter}, and so on.

Alternatively, you can define an event handler inline in the JSX:

<button onClick={function handleClick() {
  alert('You clicked me!');
}}>

Or, more concisely, using an arrow function:

<button onClick={() => {
  alert('You clicked me!');
}}>

All of these styles are equivalent. Inline event handlers are convenient for short functions.

Gotcha

Functions passed to event handlers must be passed, not called. For example:

passing a function (correct)calling a function (incorrect)
<button onClick={handleClick}><button onClick={handleClick()}>

The difference is subtle. In the first example, the handleClick function is passed as an onClick event handler. This tells React to remember it and only call your function when the user clicks the button.

In the second example, the () at the end of handleClick() fires the function immediately during rendering, without any clicks. This is because JavaScript inside the JSX { and } executes right away.

When you write code inline, the same pitfall presents itself in a different way:

passing a function (correct)calling a function (incorrect)
<button onClick={() => alert('...')}><button onClick={alert('...')}>

Passing inline code like this won’t fire on click—it fires every time the component renders:

// This alert fires when the component renders, not when clicked!
<button onClick={alert('You clicked me!')}>

If you want to define your event handler inline, wrap it in an anonymous function like so:

<button onClick={() => alert('You clicked me!')}>

Rather than executing the code inside with every render, this creates a function to be called later.

In both cases, what you want to pass is a function:

  • <button onClick={handleClick}> passes the handleClick function.
  • <button onClick={() => alert('...')}> passes the () => alert('...') function.

Check out the JavaScript Refresher for more on arrow functions.

Reading props in event handlers

Because event handlers are declared inside of a component, they have access to the component’s props. Here is a button that, when clicked, shows an alert with its message prop:

function AlertButton({ message, children }) {
  return (
    <button onClick={() => alert(message)}>
      {children}
    </button>
  );
}

export default function Toolbar() {
  return (
    <div>
      <AlertButton message="Playing!">
        Play Movie
      </AlertButton>
      <AlertButton message="Uploading!">
        Upload Image
      </AlertButton>
    </div>
  );
}

This lets these two buttons show different messages. Try changing the messages passed to them.

Passing event handlers as props

Often you’ll want the parent component to specify a child’s event handler. Consider buttons: depending on where you’re using a Button component, you might want to execute a different function—perhaps one plays a movie and another uploads an image.

To do this, pass a prop the component receives from its parent as the event handler like so:

function Button({ onClick, children }) {
  return (
    <button onClick={onClick}>
      {children}
    </button>
  );
}

function PlayButton({ movieName }) {
  function handlePlayClick() {
    alert(`Playing ${movieName}!`);
  }

  return (
    <Button onClick={handlePlayClick}>
      Play "{movieName}"
    </Button>
  );
}

function UploadButton() {
  return (
    <Button onClick={() => alert('Uploading!')}>
      Upload Image
    </Button>
  );
}

export default function Toolbar() {
  return (
    <div>
      <PlayButton movieName="Kiki's Delivery Service" />
      <UploadButton />
    </div>
  );
}

Here, the Toolbar component renders a PlayButton and an UploadButton:

  • PlayButton passes handlePlayClick as the onClick prop to the Button inside.
  • UploadButton passes () => alert('Uploading!') as the onClick prop to the Button inside.

Finally, your Button component accepts a prop called onClick. It passes that prop directly to the built-in browser <button> with onClick={onClick}. This tells React to call the passed function on click.

If you use a design system, it’s common for components like buttons to contain styling but not specify behavior. Instead, components like PlayButton and UploadButton will pass event handlers down.

Naming event handler props

Built-in components like <button> and <div> only support browser event names like onClick. However, when you’re building your own components, you can name their event handler props any way that you like.

By convention, event handler props should start with on, followed by a capital letter.

For example, the Button component’s onClick prop could have been called onSmash:

function Button({ onSmash, children }) {
  return (
    <button onClick={onSmash}>
      {children}
    </button>
  );
}

export default function App() {
  return (
    <div>
      <Button onSmash={() => alert('Playing!')}>
        Play Movie
      </Button>
      <Button onSmash={() => alert('Uploading!')}>
        Upload Image
      </Button>
    </div>
  );
}

In this example, <button onClick={onSmash}> shows that the browser <button> (lowercase) still needs a prop called onClick, but the prop name received by your custom Button component is up to you!

When your component supports multiple interactions, you might name event handler props for app-specific concepts. For example, this Toolbar component receives onPlayMovie and onUploadImage event handlers:

export default function App() {
  return (
    <Toolbar
      onPlayMovie={() => alert('Playing!')}
      onUploadImage={() => alert('Uploading!')}
    />
  );
}

function Toolbar({ onPlayMovie, onUploadImage }) {
  return (
    <div>
      <Button onClick={onPlayMovie}>
        Play Movie
      </Button>
      <Button onClick={onUploadImage}>
        Upload Image
      </Button>
    </div>
  );
}

function Button({ onClick, children }) {
  return (
    <button onClick={onClick}>
      {children}
    </button>
  );
}

Notice how the App component does not need to know what Toolbar will do with onPlayMovie or onUploadImage. That’s an implementation detail of the Toolbar. Here, Toolbar passes them down as onClick handlers to its Buttons, but it could later also trigger them on a keyboard shortcut. Naming props after app-specific interactions like onPlayMovie gives you the flexibility to change how they’re used later.

Event propagation

Event handlers will also catch events from any children your component might have. We say that an event “bubbles” or “propagates” up the tree: it starts with where the event happened, and then goes up the tree.

This <div> contains two buttons. Both the <div> and each button have their own onClick handlers. Which handlers do you think will fire when you click a button?

export default function Toolbar() {
  return (
    <div className="Toolbar" onClick={() => {
      alert('You clicked on the toolbar!');
    }}>
      <button onClick={() => alert('Playing!')}>
        Play Movie
      </button>
      <button onClick={() => alert('Uploading!')}>
        Upload Image
      </button>
    </div>
  );
}

If you click on either button, its onClick will run first, followed by the parent <div>’s onClick. So two messages will appear. If you click the toolbar itself, only the parent <div>’s onClick will run.

Gotcha

All events propagate in React except onScroll, which only works on the JSX tag you attach it to.

Stopping propagation

Event handlers receive an event object as their only argument. By convention, it’s usually called e, which stands for “event.” You can use this object to read information about the event.

That event object also lets you stop the propagation. If you want to prevent an event from reaching parent components, you need to call e.stopPropagation() like this Button component does:

function Button({ onClick, children }) {
  return (
    <button onClick={e => {
      e.stopPropagation();
      onClick();
    }}>
      {children}
    </button>
  );
}

export default function Toolbar() {
  return (
    <div className="Toolbar" onClick={() => {
      alert('You clicked on the toolbar!');
    }}>
      <Button onClick={() => alert('Playing!')}>
        Play Movie
      </Button>
      <Button onClick={() => alert('Uploading!')}>
        Upload Image
      </Button>
    </div>
  );
}

When you click on a button:

  1. React calls the onClick handler passed to <button>.
  2. That handler, defined in Button, does the following:
    • Calls e.stopPropagation(), preventing the event from bubbling further.
    • Calls the onClick function, which is a prop passed from the Toolbar component.
  3. That function, defined in the Toolbar component, displays the button’s own alert.
  4. Since the propagation was stopped, the parent <div>’s onClick handler does not run.

As a result of e.stopPropagation(), clicking on the buttons now only shows a single alert (from the <button>) rather than the two of them (from the <button> and the parent toolbar <div>). Clicking a button is not the same thing as clicking the surrounding toolbar, so stopping the propagation makes sense for this UI.

Deep Dive

Capture phase events

Passing handlers as alternative to propagation

Notice how this click handler runs a line of code and then calls the onClick prop passed by the parent:

function Button({ onClick, children }) {
  return (
    <button onClick={e => {
      e.stopPropagation();
      onClick();
    }}>
      {children}
    </button>
  );
}

You could add more code to this handler before calling the parent onClick event handler, too. This pattern provides an alternative to propagation. It lets the child component handle the event, while also letting the parent component specify some additional behavior. Unlike propagation, it’s not automatic. But the benefit of this pattern is that you can clearly follow the whole chain code that executes as a result of some event.

If you rely on propagation and it’s difficult to trace which handlers execute and why, try this approach instead.

Preventing default behavior

Some browser events have default behavior associated with them. For example, a <form> submit event, which happens when a button inside of it is clicked, will reload the whole page by default:

export default function Signup() {
  return (
    <form onSubmit={() => alert('Submitting!')}>
      <input />
      <button>Send</button>
    </form>
  );
}

You can call e.preventDefault() on the event object to stop this from happening:

export default function Signup() {
  return (
    <form onSubmit={e => {
      e.preventDefault();
      alert('Submitting!');
    }}>
      <input />
      <button>Send</button>
    </form>
  );
}

Don’t confuse e.stopPropagation() and e.preventDefault(). They are both useful, but are unrelated:

Can event handlers have side effects?

Absolutely! Event handlers are the best place for side effects.

Unlike rendering functions, event handlers don’t need to be pure, so it’s a great place to change something—for example, change an input’s value in response to typing, or change a list in response to a button press. However, in order to change some information, you first need some way to store it. In React, this is done by using state, a component’s memory. You will learn all about it on the next page.

Recap

  • You can handle events by passing a function as a prop to an element like <button>.
  • Event handlers must be passed, not called! onClick={handleClick}, not onClick={handleClick()}.
  • You can define an event handler function separately or inline.
  • Event handlers are defined inside a component, so they can access props.
  • You can declare an event handler in a parent and pass it as a prop to a child.
  • You can define your own event handler props with application-specific names.
  • Events propagate upwards. Call e.stopPropagation() on the first argument to prevent that.
  • Events may have unwanted default browser behavior. Call e.preventDefault() to prevent that.
  • Explicitly calling an event handler prop from a child handler is a good alternative to propagation.

Challenge 1 of 2:
Fix an event handler

Clicking this button is supposed to switch the page background between white and black. However, nothing happens when you click it. Fix the problem. (Don’t worry about the logic inside handleClick—that part is fine.)

export default function LightSwitch() {
  function handleClick() {
    let bodyStyle = document.body.style;
    if (bodyStyle.backgroundColor === 'black') {
      bodyStyle.backgroundColor = 'white';
    } else {
      bodyStyle.backgroundColor = 'black';
    }
  }

  return (
    <button onClick={handleClick()}>
      Toggle the lights
    </button>
  );
}