Introduction to events
======================

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* Overview: Building blocks
* Next

Events are things that happen in the system you are programming, which the system tells you about so your code can react to them.

 For example, if the user clicks a button on a webpage, you might want to react to that action by displaying an information box.
 In this article, we discuss some important concepts surrounding events, and look at how they work in browsers.
 This won't be an exhaustive study; just what you need to know at this stage.


|  |  |
| --- | --- |
| Prerequisites: | 
 A basic understanding of HTML, CSS, and
 JavaScript first steps.
  |
| Objective: | 
 To understand the fundamental theory of events, how they work in
 browsers, and how events may differ in different programming
 environments.
  |

What is an event?
-----------------

 Events are things that happen in the system you are programming — the system produces (or "fires") a signal of some kind when an event occurs, and provides a mechanism by which an action can be automatically taken (that is, some code running) when the event occurs.
 Events are fired inside the browser window, and tend to be attached to a specific item that resides in it. This might be a single element, a set of elements, the HTML document loaded in the current tab, or the entire browser window.
 There are many different types of events that can occur.

For example:

* The user selects, clicks, or hovers the cursor over a certain element.
* The user chooses a key on the keyboard.
* The user resizes or closes the browser window.
* A web page finishes loading.
* A form is submitted.
* A video is played, paused, or ends.
* An error occurs.

You can gather from this (and from glancing at the MDN event reference) that there are **a lot** of events that can be fired.

 To react to an event, you attach an **event handler** to it. This is a block of code (usually a JavaScript function that you as a programmer create) that runs when the event fires.
 When such a block of code is defined to run in response to an event, we say we are **registering an event handler**.
 Note: Event handlers are sometimes called **event listeners** — they are pretty much interchangeable for our purposes, although strictly speaking, they work together.
 The listener listens out for the event happening, and the handler is the code that is run in response to it happening.

**Note:** Web events are not part of the core JavaScript language — they are defined as part of the APIs built into the browser.

### An example: handling a click event

In the following example, we have a single `<button>` in the page:


```html
<button>Change color</button>
```


```
button {
  margin: 10px;
}
```


Then we have some JavaScript. We'll look at this in more detail in the next section, but for now we can just say: it adds an event handler to the button's `"click"` event, and the handler reacts to the event by setting the page background to a random color:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

btn.addEventListener("click", () => {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
});
```


The example output is as follows. Try clicking the button:

Using addEventListener()
------------------------

As we saw in the last example, objects that can fire events have an `addEventListener()` method, and this is the recommended mechanism for adding event handlers.

Let's take a closer look at the code from the last example:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

btn.addEventListener("click", () => {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
});
```


The HTML `<button>` element will fire an event when the user clicks the button. So it defines an `addEventListener()` function, which we are calling here. We're passing in two parameters:

* the string `"click"`, to indicate that we want to listen to the click event. Buttons can fire lots of other events, such as `"mouseover"` when the user moves their mouse over the button, or `"keydown"` when the user presses a key and the button is focused.
* a function to call when the event happens. In our case, the function generates a random RGB color and sets the `background-color` of the page `<body>` to that color.

It is fine to make the handler function a separate named function, like this:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

function changeBackground() {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
}

btn.addEventListener("click", changeBackground);
```

### Listening for other events

There are many different events that can be fired by a button element. Let's experiment.

 First, make a local copy of random-color-addeventlistener.html, and open it in your browser.
 It's just a copy of the simple random color example we've played with already. Now try changing `click` to the following different values in turn, and observing the results in the example:

* `focus` and `blur` — The color changes when the button is focused and unfocused; try pressing the tab to focus on the button and press the tab again to focus away from the button.
 These are often used to display information about filling in form fields when they are focused, or to display an error message if a form field is filled with an incorrect value.
* `dblclick` — The color changes only when the button is double-clicked.
* `mouseover` and `mouseout` — The color changes when the mouse pointer hovers over the button, or when the pointer moves off the button, respectively.

Some events, such as `click`, are available on nearly any element. Others are more specific and only useful in certain situations: for example, the `play` event is only available on some elements, such as `<video>`.

### Removing listeners

If you've added an event handler using `addEventListener()`, you can remove it again using the `removeEventListener()` method. For example, this would remove the `changeBackground()` event handler:


```js
btn.removeEventListener("click", changeBackground);
```


 Event handlers can also be removed by passing an `AbortSignal` to `addEventListener()` and then later calling `abort()` on the controller owning the `AbortSignal`.
 For example, to add an event handler that we can remove with an `AbortSignal`:


```js
const controller = new AbortController();

btn.addEventListener("click",
  () => {
    const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
    document.body.style.backgroundColor = rndCol;
  },
  { signal: controller.signal } // pass an AbortSignal to this handler
);
```


Then the event handler created by the code above can be removed like this:


```js
controller.abort(); // removes any/all event handlers associated with this controller
```


 For simple, small programs, cleaning up old, unused event handlers isn't necessary, but for larger, more complex programs, it can improve efficiency.
 Also, the ability to remove event handlers allows you to have the same button performing different actions in different circumstances: all you have to do is add or remove handlers.

### Adding multiple listeners for a single event

By making more than one call to `addEventListener()`, providing different handlers, you can have multiple handlers for a single event:


```js
myElement.addEventListener("click", functionA);
myElement.addEventListener("click", functionB);
```


Both functions would now run when the element is clicked.

### Learn more

There are other powerful features and options available with `addEventListener()`.

These are a little out of scope for this article, but if you want to read them, visit the `addEventListener()` and `removeEventListener()` reference pages.

Other event listener mechanisms
-------------------------------

We recommend that you use `addEventListener()` to register event handlers. It's the most powerful method and scales best with more complex programs. However, there are two other ways of registering event handlers that you might see: *event handler properties* and *inline event handlers*.

### Event handler properties

 Objects (such as buttons) that can fire events also usually have properties whose name is `on` followed by the name of the event. For example, elements have a property `onclick`.
 This is called an *event handler property*. To listen for the event, you can assign the handler function to the property.

For example, we could rewrite the random-color example like this:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

btn.onclick = () => {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
};
```


You can also set the handler property to a named function:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

function bgChange() {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
}

btn.onclick = bgChange;
```


With event handler properties, you can't add more than one handler for a single event. For example, you can call `addEventListener('click', handler)` on an element multiple times, with different functions specified in the second argument:


```js
element.addEventListener("click", function1);
element.addEventListener("click", function2);
```


This is impossible with event handler properties because any subsequent attempts to set the property will overwrite earlier ones:


```js
element.onclick = function1;
element.onclick = function2;
```

### Inline event handlers — don't use these

You might also see a pattern like this in your code:


```html
<button onclick="bgChange()">Press me</button>
```



```js
function bgChange() {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
}
```


 The earliest method of registering event handlers found on the Web involved *event handler HTML attributes* (or *inline event handlers*) like the one shown above — the attribute value is literally the JavaScript code you want to run when the event occurs.
 The above example invokes a function defined inside a `<script>` element on the same page, but you could also insert JavaScript directly inside the attribute, for example:


```html
<button onclick="alert('Hello, this is my old-fashioned event handler!');">
  Press me
</button>
```


 You can find HTML attribute equivalents for many of the event handler properties; however, you shouldn't use these — they are considered bad practice.
 It might seem easy to use an event handler attribute if you are doing something really quick, but they quickly become unmanageable and inefficient.

For a start, it is not a good idea to mix up your HTML and your JavaScript, as it becomes hard to read. Keeping your JavaScript separate is a good practice, and if it is in a separate file you can apply it to multiple HTML documents.

 Even in a single file, inline event handlers are not a good idea.
 One button is OK, but what if you had 100 buttons? You'd have to add 100 attributes to the file; it would quickly turn into a maintenance nightmare.
 With JavaScript, you could easily add an event handler function to all the buttons on the page no matter how many there were, using something like this:


```js
const buttons = document.querySelectorAll("button");

for (const button of buttons) {
  button.addEventListener("click", bgChange);
}
```


Finally, many common server configurations will disallow inline JavaScript, as a security measure.

**You should never use the HTML event handler attributes** — those are outdated, and using them is bad practice.

Event objects
-------------

 Sometimes, inside an event handler function, you'll see a parameter specified with a name such as `event`, `evt`, or `e`.
 This is called the **event object**, and it is automatically passed to event handlers to provide extra features and information.
 For example, let's rewrite our random color example again slightly:


```js
const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() \* (number + 1));
}

function bgChange(e) {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  e.target.style.backgroundColor = rndCol;
  console.log(e);
}

btn.addEventListener("click", bgChange);
```


**Note:** You can find the full source code for this example on GitHub (also see it running live).

 Here you can see we are including an event object, **e**, in the function, and in the function setting a background color style on `e.target` — which is the button itself.
 The `target` property of the event object is always a reference to the element the event occurred upon.
 So, in this example, we are setting a random background color on the button, not the page.

**Note:** See the Event delegation section below for an example where we use `event.target`.


**Note:** You can use any name you like for the event object — you just need to choose a name that you can then use to reference it inside the event handler function.
 `e`/`evt`/`event` is most commonly used by developers because they are short and easy to remember.
 It's always good to be consistent — with yourself, and with others if possible.


### Extra properties of event objects

Most event objects have a standard set of properties and methods available on the event object; see the `Event` object reference for a full list.

Some event objects add extra properties that are relevant to that particular type of event. For example, the `keydown` event fires when the user presses a key. Its event object is a `KeyboardEvent`, which is a specialized `Event` object with a `key` property that tells you which key was pressed:


```html
<input id="textBox" type="text" />
<div id="output"></div>
```



```js
const textBox = document.querySelector("#textBox");
const output = document.querySelector("#output");
textBox.addEventListener("keydown", (event) => {
  output.textContent = `You pressed "${event.key}".`;
});
```


```
div {
  margin: 0.5rem 0;
}
```


Try typing into the text box and see the output:

Preventing default behavior
---------------------------

 Sometimes, you'll come across a situation where you want to prevent an event from doing what it does by default.
 The most common example is that of a web form, for example, a custom registration form.
 When you fill in the details and click the submit button, the natural behavior is for the data to be submitted to a specified page on the server for processing, and the browser to be redirected to a "success message" page of some kind (or the same page, if another is not specified).

 The trouble comes when the user has not submitted the data correctly — as a developer, you want to prevent the submission to the server and give an error message saying what's wrong and what needs to be done to put things right.
 Some browsers support automatic form data validation features, but since many don't, you are advised to not rely on those and implement your own validation checks.
 Let's look at a simple example.

First, a simple HTML form that requires you to enter your first and last name:


```html
<form>
  <div>
    <label for="fname">First name: </label>
    <input id="fname" type="text" />
  </div>
  <div>
    <label for="lname">Last name: </label>
    <input id="lname" type="text" />
  </div>
  <div>
    <input id="submit" type="submit" />
  </div>
</form>
<p></p>
```


```
div {
  margin-bottom: 10px;
}
```


 Now some JavaScript — here we implement a very simple check inside a handler for the `submit` event (the submit event is fired on a form when it is submitted) that tests whether the text fields are empty.
 If they are, we call the `preventDefault()` function on the event object — which stops the form submission — and then display an error message in the paragraph below our form to tell the user what's wrong:


```js
const form = document.querySelector("form");
const fname = document.getElementById("fname");
const lname = document.getElementById("lname");
const para = document.querySelector("p");

form.addEventListener("submit", (e) => {
  if (fname.value === "" || lname.value === "") {
    e.preventDefault();
    para.textContent = "You need to fill in both names!";
  }
});
```


 Obviously, this is pretty weak form validation — it wouldn't stop the user from validating the form with spaces or numbers entered into the fields, for example — but it is OK for example purposes.
 The output is as follows:

**Note:** For the full source code, see preventdefault-validation.html (also see it running live here).

Event bubbling
--------------

Event bubbling describes how the browser handles events targeted at nested elements.

### Setting a listener on a parent element

Consider a web page like this:


```html
<div id="container">
  <button>Click me!</button>
</div>
<pre id="output"></pre>
```


Here the button is inside another element, a `<div>` element. We say that the `<div>` element here is the **parent** of the element it contains. What happens if we add a click event handler to the parent, then click the button?


```js
const output = document.querySelector("#output");
function handleClick(e) {
  output.textContent += `You clicked on a ${e.currentTarget.tagName} element\n`;
}

const container = document.querySelector("#container");
container.addEventListener("click", handleClick);
```


You'll see that the parent fires a click event when the user clicks the button:
```

You clicked on a DIV element
```


This makes sense: the button is inside the `<div>`, so when you click the button you're also implicitly clicking the element it is inside.

### Bubbling example

What happens if we add event listeners to the button *and* the parent?


```html
<body>
  <div id="container">
    <button>Click me!</button>
  </div>
  <pre id="output"></pre>
</body>
```


Let's try adding click event handlers to the button, its parent (the `<div>`), and the `<body>` element that contains both of them:


```js
const output = document.querySelector("#output");
function handleClick(e) {
  output.textContent += `You clicked on a ${e.currentTarget.tagName} element\n`;
}

const container = document.querySelector("#container");
const button = document.querySelector("button");

document.body.addEventListener("click", handleClick);
container.addEventListener("click", handleClick);
button.addEventListener("click", handleClick);
```


You'll see that all three elements fire a click event when the user clicks the button:
```

You clicked on a BUTTON element
You clicked on a DIV element
You clicked on a BODY element
```


In this case:

* the click on the button fires first
* followed by the click on its parent (the `<div>` element)
* followed by the `<div>` element's parent (the `<body>` element).

We describe this by saying that the event **bubbles up** from the innermost element that was clicked.

This behavior can be useful and can also cause unexpected problems. In the next sections, we'll see a problem that it causes, and find the solution.

### Video player example

In this example our page contains a video, which is hidden initially, and a button labeled "Display video". We want the following interaction:

* When the user clicks the "Display video" button, show the box containing the video, but don't start playing the video yet.
* When the user clicks on the video, start playing the video.
* When the user clicks anywhere in the box outside the video, hide the box.

The HTML looks like this:


```html
<button>Display video</button>

<div class="hidden">
  <video>
    <source
 src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.webm"
 type="video/webm" />
    <p>
      Your browser doesn't support HTML video. Here is a
      <a href="rabbit320.mp4">link to the video</a> instead.
    </p>
  </video>
</div>
```


It includes:

* a `<button>` element
* a `<div>` element which initially has a `class="hidden"` attribute
* a `<video>` element nested inside the `<div>` element.

We're using CSS to hide elements with the `"hidden"` class set.
```

div {
  width: 100%;
  height: 100%;
  background-color: #eee;
}

.hidden {
  display: none;
}

div video {
  padding: 40px;
  display: block;
  width: 400px;
  margin: 40px auto;
}
```


The JavaScript looks like this:


```js
const btn = document.querySelector("button");
const box = document.querySelector("div");
const video = document.querySelector("video");

btn.addEventListener("click", () => box.classList.remove("hidden"));
video.addEventListener("click", () => video.play());
box.addEventListener("click", () => box.classList.add("hidden"));
```


This adds three `'click'` event listeners:

* one on the `<button>`, which shows the `<div>` that contains the `<video>`
* one on the `<video>`, which starts playing the video
* one on the `<div>`, which hides the video

Let's see how this works:

You should see that when you click the button, the box and the video it contains are shown. But then when you click the video, the video starts to play, but the box is hidden again!

The video is inside the `<div>` — it is part of it — so clicking the video runs *both* the event handlers, causing this behavior.

### Fixing the problem with stopPropagation()

 As we saw in the last section, event bubbling can sometimes create problems, but there is a way to prevent it.
 The `Event` object has a function available on it called `stopPropagation()` which, when called inside an event handler, prevents the event from bubbling up to any other elements.

We can fix our current problem by changing the JavaScript to this:


```js
const btn = document.querySelector("button");
const box = document.querySelector("div");
const video = document.querySelector("video");

btn.addEventListener("click", () => box.classList.remove("hidden"));

video.addEventListener("click", (event) => {
  event.stopPropagation();
  video.play();
});

box.addEventListener("click", () => box.classList.add("hidden"));
```


All we're doing here is calling `stopPropagation()` on the event object in the handler for the `<video>` element's `'click'` event. This will stop that event from bubbling up to the box. Now try clicking the button and then the video:
```

<button>Display video</button>

<div class="hidden">
  <video>
    <source
 src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.webm"
 type="video/webm" />
    <p>
      Your browser doesn't support HTML video. Here is a
      <a href="rabbit320.mp4">link to the video</a> instead.
    </p>
  </video>
</div>
```


```
div {
  width: 100%;
  height: 100%;
  background-color: #eee;
}

.hidden {
  display: none;
}

div video {
  padding: 40px;
  display: block;
  width: 400px;
  margin: 40px auto;
}
```

### Event capture

An alternative form of event propagation is *event capture*. This is like event bubbling but the order is reversed: so instead of the event firing first on the innermost element targeted, and then on successively less nested elements, the event fires first on the *least nested* element, and then on successively more nested elements, until the target is reached.

Event capture is disabled by default. To enable it you have to pass the `capture` option in `addEventListener()`.

This example is just like the bubbling example we saw earlier, except that we have used the `capture` option:


```html
<body>
  <div id="container">
    <button>Click me!</button>
  </div>
  <pre id="output"></pre>
</body>
```



```js
const output = document.querySelector("#output");
function handleClick(e) {
  output.textContent += `You clicked on a ${e.currentTarget.tagName} element\n`;
}

const container = document.querySelector("#container");
const button = document.querySelector("button");

document.body.addEventListener("click", handleClick, { capture: true });
container.addEventListener("click", handleClick, { capture: true });
button.addEventListener("click", handleClick);
```


In this case, the order of messages is reversed: the `<body>` event handler fires first, followed by the `<div>` event handler, followed by the `<button>` event handler:
```

You clicked on a BODY element
You clicked on a DIV element
You clicked on a BUTTON element
```


Why bother with both capturing and bubbling? In the bad old days, when browsers were much less cross-compatible than now, Netscape only used event capturing, and Internet Explorer used only event bubbling. When the W3C decided to try to standardize the behavior and reach a consensus, they ended up with this system that included both, which is what modern browsers implement.

By default almost all event handlers are registered in the bubbling phase, and this makes more sense most of the time.

Event delegation
----------------

In the last section, we looked at a problem caused by event bubbling and how to fix it. Event bubbling isn't just annoying, though: it can be very useful. In particular, it enables **event delegation**. In this practice, when we want some code to run when the user interacts with any one of a large number of child elements, we set the event listener on their parent and have events that happen on them bubble up to their parent rather than having to set the event listener on every child individually.

Let's go back to our first example, where we set the background color of the whole page when the user clicked a button. Suppose that instead, the page is divided into 16 tiles, and we want to set each tile to a random color when the user clicks that tile.

Here's the HTML:


```html
<div id="container">
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
  <div class="tile"></div>
</div>
```


We have a little CSS, to set the size and position of the tiles:


```css
.tile {
  height: 100px;
  width: 25%;
  float: left;
}
```


Now in JavaScript, we could add a click event handler for every tile. But a much simpler and more efficient option is to set the click event handler on the parent, and rely on event bubbling to ensure that the handler is executed when the user clicks on a tile:


```js
function random(number) {
  return Math.floor(Math.random() \* number);
}

function bgChange() {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  return rndCol;
}

const container = document.querySelector("#container");

container.addEventListener("click", (event) => {
  event.target.style.backgroundColor = bgChange();
});
```


The output is as follows (try clicking around on it):

**Note:** In this example, we're using `event.target` to get the element that was the target of the event (that is, the innermost element). If we wanted to access the element that handled this event (in this case the container) we could use `event.currentTarget`.

**Note:** See useful-eventtarget.html for the full source code; also see it running live here.

It's not just web pages
-----------------------

 Events are not unique to JavaScript — most programming languages have some kind of event model, and the way the model works often differs from JavaScript's way.
 In fact, the event model in JavaScript for web pages differs from the event model for JavaScript as it is used in other environments.

 For example, Node.js is a very popular JavaScript runtime that enables developers to use JavaScript to build network and server-side applications.
 The Node.js event model relies on listeners to listen for events and emitters to emit events periodically — it doesn't sound that different, but the code is quite different, making use of functions like `on()` to register an event listener, and `once()` to register an event listener that unregisters after it has run once.
 The HTTP connect event docs provide a good example.

 You can also use JavaScript to build cross-browser add-ons — browser functionality enhancements — using a technology called WebExtensions.
 The event model is similar to the web events model, but a bit different — event listeners' properties are written in camel case (such as `onMessage` rather than `onmessage`), and need to be combined with the `addListener` function.
 See the `runtime.onMessage` page for an example.

You don't need to understand anything about other such environments at this stage in your learning; we just wanted to make it clear that events can differ in different programming environments.

Test your skills!
-----------------

You've reached the end of this article, but can you remember the most important information? To verify you've retained this information before you move on — see Test your skills: Events.

Conclusion
----------

 You should now know all you need to know about web events at this early stage.
 As mentioned, events are not really part of the core JavaScript — they are defined in browser Web APIs.

 Also, it is important to understand that the different contexts in which JavaScript is used have different event models — from Web APIs to other areas such as browser WebExtensions and Node.js (server-side JavaScript).
 We are not expecting you to understand all of these areas now, but it certainly helps to understand the basics of events as you forge ahead with learning web development.

**Note:** If you get stuck, you can reach out to us in one of our communication channels.

See also
--------

* domevents.dev — a very useful interactive playground app that enables learning about the behavior of the DOM Event system through exploration.
* Event reference
* Event order (discussion of capturing and bubbling) — an excellently detailed piece by Peter-Paul Koch.
* Event accessing (discussion of the event object) — another excellently detailed piece by Peter-Paul Koch.
* Previous
* Overview: Building blocks
* Next