EventTarget.addEventListener()

The addEventListener() method of the EventTarget interface sets up a function that will be called whenever the specified event is delivered to the target.

Common targets are Element, or its children, Document, and Window, but the target may be any object that supports events (such as XMLHttpRequest).

The method addEventListener() works by adding a function, or an object that implements EventListener, to the list of event listeners for the specified event type on the EventTarget on which it's called. If the function or object is already in the list of event listeners for this target, the function or object is not added a second time.

If an event listener is added to an EventTarget from inside another listener — that is, during the processing of the event — that event will not trigger the new listener. However, the new listener may be triggered during a later stage of event flow, such as during the bubbling phase.

type

A case-sensitive string representing the event type to listen for.

listener

The object that receives a notification (an object that implements the Event interface) when an event of the specified type occurs. This must be an object implementing the EventListener interface, or a JavaScript function. See The event listener callback for details on the callback itself.

options Optional

An object that specifies characteristics about the event listener. The available options are:

capture

A boolean value indicating that events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree. If not specified, defaults to false.

once

A boolean value indicating that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked. If not specified, defaults to false.

passive

A boolean value that, if true, indicates that the function specified by listener will never call preventDefault(). If a passive listener does call preventDefault(), the user agent will do nothing other than generate a console warning. If not specified, defaults to false – except that in browsers other than Safari and Internet Explorer, defaults to true for the wheel, mousewheel, touchstart and touchmove events. See Improving scrolling performance with passive listeners to learn more.

signal

An AbortSignal. The listener will be removed when the given AbortSignal object's abort() method is called. If not specified, no AbortSignal is associated with the listener.

useCapture Optional

A boolean value indicating whether events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree. Events that are bubbling upward through the tree will not trigger a listener designated to use capture. Event bubbling and capturing are two ways of propagating events that occur in an element that is nested within another element, when both elements have registered a handle for that event. The event propagation mode determines the order in which elements receive the event. See DOM Level 3 Events and JavaScript Event order for a detailed explanation. If not specified, useCapture defaults to false.

Note: For event listeners attached to the event target, the event is in the target phase, rather than the capturing and bubbling phases. Event listeners in the capturing phase are called before event listeners in any non-capturing phases.

wantsUntrusted Optional

A Firefox (Gecko)-specific parameter. If true, the listener receives synthetic events dispatched by web content (the default is false for browser chrome and true for regular web pages). This parameter is useful for code found in add-ons, as well as the browser itself.

None.

The event listener can be specified as either a callback function or an object that implements EventListener, whose handleEvent() method serves as the callback function.

The callback function itself has the same parameters and return value as the handleEvent() method; that is, the callback accepts a single parameter: an object based on Event describing the event that has occurred, and it returns nothing.

For example, an event handler callback that can be used to handle both fullscreenchange and fullscreenerror might look like this:

function eventHandler(event) {
  if (event.type == 'fullscreenchange') {
    /* handle a full screen toggle */
  } else /* fullscreenerror */ {
    /* handle a full screen toggle error */
  }
}

In older versions of the DOM specification, the third parameter of addEventListener() was a Boolean value indicating whether or not to use capture. Over time, it became clear that more options were needed. Rather than adding more parameters to the function (complicating things enormously when dealing with optional values), the third parameter was changed to an object that can contain various properties defining the values of options to configure the process of removing the event listener.

Because older browsers (as well as some not-too-old browsers) still assume the third parameter is a Boolean, you need to build your code to handle this scenario intelligently. You can do this by using feature detection for each of the options you're interested in.

For example, if you want to check for the passive option:

let passiveSupported = false;

try {
  const options = {
    get passive() { // This function will be called when the browser
                    //   attempts to access the passive property.
      passiveSupported = true;
      return false;
    }
  };

  window.addEventListener("test", null, options);
  window.removeEventListener("test", null, options);
} catch(err) {
  passiveSupported = false;
}

This creates an options object with a getter function for the passive property; the getter sets a flag, passiveSupported, to true if it gets called. That means that if the browser checks the value of the passive property on the options object, passiveSupported will be set to true; otherwise, it will remain false. We then call addEventListener() to set up a fake event handler, specifying those options, so that the options will be checked if the browser recognizes an object as the third parameter. Then, we call removeEventListener() to clean up after ourselves. (Note that handleEvent() is ignored on event listeners that aren't called.)

You can check whether any option is supported this way. Just add a getter for that option using code similar to what is shown above.

Then, when you want to create an actual event listener that uses the options in question, you can do something like this:

someElement.addEventListener("mouseup", handleMouseUp, passiveSupported
                               ? { passive: true } : false);

Here we're adding a listener for the mouseup event on the element someElement. For the third parameter, if passiveSupported is true, we're specifying an options object with passive set to true; otherwise, we know that we need to pass a Boolean, and we pass false as the value of the useCapture parameter.

If you'd prefer, you can use a third-party library like Modernizr or Detect It to do this test for you.

You can learn more from the article about EventListenerOptions from the Web Incubator Community Group.

This example demonstrates how to use addEventListener() to watch for mouse clicks on an element.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText() {
  const t2 = document.getElementById("t2");
  if (t2.firstChild.nodeValue == "three") {
    t2.firstChild.nodeValue = "two";
  } else {
    t2.firstChild.nodeValue = "three";
  }
}

// Add event listener to table
const el = document.getElementById("outside");
el.addEventListener("click", modifyText, false);

In this code, modifyText() is a listener for click events registered using addEventListener(). A click anywhere in the table bubbles up to the handler and runs modifyText().

Result

This example demonstrates how to add an addEventListener() that can be aborted with an AbortSignal.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Add an abortable event listener to table
const controller = new AbortController();
const el = document.getElementById("outside");
el.addEventListener("click", modifyText, { signal: controller.signal } );

// Function to change the content of t2
function modifyText() {
  const t2 = document.getElementById("t2");
  if (t2.firstChild.nodeValue == "three") {
    t2.firstChild.nodeValue = "two";
  } else {
    t2.firstChild.nodeValue = "three";
    controller.abort(); // remove listener after value reaches "three"
  }
}

In the example above, we modify the code in the previous example such that after the second row's content changes to "three", we call abort() from the AbortController we passed to the addEventListener() call. That results in the value remaining as "three" forever because we no longer have any code listening for a click event.

Result

Here, we'll take a look at how to use an anonymous function to pass parameters into the event listener.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText(new_text) {
  const t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;
}

// Function to add event listener to table
const el = document.getElementById("outside");
el.addEventListener("click", function(){modifyText("four")}, false);

Notice that the listener is an anonymous function that encapsulates code that is then, in turn, able to send parameters to the modifyText() function, which is responsible for actually responding to the event.

Result

This example demonstrates a simple event listener implemented using arrow function notation.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText(new_text) {
  const t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;
}

// Add event listener to table with an arrow function
const el = document.getElementById("outside");
el.addEventListener("click", () => { modifyText("four"); }, false);

Result

Please note that while anonymous and arrow functions are similar, they have different this bindings. While anonymous (and all traditional JavaScript functions) create their own this bindings, arrow functions inherit the this binding of the containing function.

That means that the variables and constants available to the containing function are also available to the event handler when using an arrow function.

HTML

<div class="outer">
  outer, once & none-once
  <div class="middle" target="_blank">
    middle, capture & none-capture
    <a class="inner1" href="https://www.mozilla.org" target="_blank">
      inner1, passive & preventDefault(which is not allowed)
    </a>
    <a class="inner2" href="https://developer.mozilla.org/" target="_blank">
      inner2, none-passive & preventDefault(not open new page)
    </a>
  </div>
</div>

CSS

.outer, .middle, .inner1, .inner2 {
  display: block;
  width:   520px;
  padding: 15px;
  margin:  15px;
  text-decoration: none;
}
.outer {
  border: 1px solid red;
  color:  red;
}
.middle {
  border: 1px solid green;
  color:  green;
  width:  460px;
}
.inner1, .inner2 {
  border: 1px solid purple;
  color:  purple;
  width:  400px;
}

JavaScript

const outer  = document.querySelector('.outer');
const middle = document.querySelector('.middle');
const inner1 = document.querySelector('.inner1');
const inner2 = document.querySelector('.inner2');

const capture = {
  capture : true
};
const noneCapture = {
  capture : false
};
const once = {
  once : true
};
const noneOnce = {
  once : false
};
const passive = {
  passive : true
};
const nonePassive = {
  passive : false
};

outer.addEventListener('click', onceHandler, once);
outer.addEventListener('click', noneOnceHandler, noneOnce);
middle.addEventListener('click', captureHandler, capture);
middle.addEventListener('click', noneCaptureHandler, noneCapture);
inner1.addEventListener('click', passiveHandler, passive);
inner2.addEventListener('click', nonePassiveHandler, nonePassive);

function onceHandler(event) {
  alert('outer, once');
}
function noneOnceHandler(event) {
  alert('outer, none-once, default');
}
function captureHandler(event) {
  //event.stopImmediatePropagation();
  alert('middle, capture');
}
function noneCaptureHandler(event) {
  alert('middle, none-capture, default');
}
function passiveHandler(event) {
  // Unable to preventDefault inside passive event listener invocation.
  event.preventDefault();
  alert('inner1, passive, open new page');
}
function nonePassiveHandler(event) {
  event.preventDefault();
  //event.stopPropagation();
  alert('inner2, none-passive, default, not open new page');
}

Result

Click the outer, middle, inner containers respectively to see how the options work.

Before using a particular value in the options object, it's a good idea to ensure that the user's browser supports it, since these are an addition that not all browsers have supported historically. See Safely detecting option support for details.

It is often desirable to reference the element on which the event handler was fired, such as when using a generic handler for a set of similar elements.

When attaching a handler function to an element using addEventListener(), the value of this inside the handler will be a reference to the element. It will be the same as the value of the currentTarget property of the event argument that is passed to the handler.

my_element.addEventListener('click', function (e) {
  console.log(this.className)           // logs the className of my_element
  console.log(e.currentTarget === this) // logs `true`
})

As a reminder, arrow functions do not have their own this context.

my_element.addEventListener('click', (e) => {
  console.log(this.className)           // WARNING: `this` is not `my_element`
  console.log(e.currentTarget === this) // logs `false`
})

If an event handler (for example, onclick) is specified on an element in the HTML source, the JavaScript code in the attribute value is effectively wrapped in a handler function that binds the value of this in a manner consistent with the addEventListener(); an occurrence of this within the code represents a reference to the element.

<table id="my_table" onclick="console.log(this.id);"><!-- `this` refers to the table; logs 'my_table' -->
  ...
</table>

Note that the value of this inside a function, called by the code in the attribute value, behaves as per standard rules. This is shown in the following example:

<script>
  function logID() { console.log(this.id); }
</script>
<table id="my_table" onclick="logID();"><!-- when called, `this` will refer to the global object -->
  ...
</table>

The value of this within logID() is a reference to the global object Window (or undefined in the case of strict mode.

Specifying "this" using bind()

The Function.prototype.bind() method lets you establish a fixed this context for all subsequent calls — bypassing problems where it's unclear what this will be, depending on the context from which your function was called. Note, however, that you'll need to keep a reference to the listener around so you can remove it later.

This is an example with and without bind():

const Something = function(element) {
  // |this| is a newly created object
  this.name = 'Something Good';
  this.onclick1 = function(event) {
    console.log(this.name); // undefined, as |this| is the element
  };

  this.onclick2 = function(event) {
    console.log(this.name); // 'Something Good', as |this| is bound to newly created object
  };

  // bind causes a fixed `this` context to be assigned to onclick2
  this.onclick2 = this.onclick2.bind(this);

  element.addEventListener('click', this.onclick1, false);
  element.addEventListener('click', this.onclick2, false); // Trick
}
const s = new Something(document.body);

Another solution is using a special function called handleEvent() to catch any events:

const Something = function(element) {
  // |this| is a newly created object
  this.name = 'Something Good';
  this.handleEvent = function(event) {
    console.log(this.name); // 'Something Good', as this is bound to newly created object
    switch(event.type) {
      case 'click':
        // some code here...
        break;
      case 'dblclick':
        // some code here...
        break;
    }
  };

  // Note that the listeners in this case are |this|, not this.handleEvent
  element.addEventListener('click', this, false);
  element.addEventListener('dblclick', this, false);

  // You can properly remove the listeners
  element.removeEventListener('click', this, false);
  element.removeEventListener('dblclick', this, false);
}
const s = new Something(document.body);

Another way of handling the reference to this is to pass to the EventListener a function that calls the method of the object that contains the fields that need to be accessed:

class SomeClass {

  constructor() {
    this.name = 'Something Good';
  }

  register() {
    const that = this;
    window.addEventListener('keydown', function(e) { that.someMethod(e); });
  }

  someMethod(e) {
    console.log(this.name);
    switch(e.keyCode) {
      case 5:
        // some code here...
        break;
      case 6:
        // some code here...
        break;
    }
  }

}

const myObject = new SomeClass();
myObject.register();

It may seem that event listeners are like islands, and that it is extremely difficult to pass them any data, much less to get any data back from them after they execute. Event listeners only take one argument, the Event Object, which is automatically passed to the listener, and the return value is ignored. So how can we get data in and back out of them? There are a number of good methods for doing this.

Getting data into an event listener using "this"

As mentioned above, you can use Function.prototype.bind() to pass a value to an event listener via the this reference variable.

const myButton = document.getElementById('my-button-id');
const someString = 'Data';

myButton.addEventListener('click', function () {
  console.log(this); // Expected Value: 'Data'
}.bind(someString));

This method is suitable when you don't need to know which HTML element the event listener fired on programmatically from within the event listener. The primary benefit to doing this is that the event listener receives the data in much the same way that it would if you were to actually pass it through its argument list.

Getting data into an event listener using the outer scope property

When an outer scope contains a variable declaration (with const, let), all the inner functions declared in that scope have access to that variable (look here for information on outer/inner functions, and here for information on variable scope). Therefore, one of the simplest ways to access data from outside of an event listener is to make it accessible to the scope in which the event listener is declared.

const myButton = document.getElementById('my-button-id');
let someString = 'Data';

myButton.addEventListener('click', function() {
  console.log(someString);  // Expected Value: 'Data'

  someString = 'Data Again';
});

console.log(someString);  // Expected Value: 'Data' (will never output 'Data Again')

Getting data into and out of an event listener using objects

Unlike most functions in JavaScript, objects are retained in memory as long as a variable referencing them exists in memory. This, and the fact that objects can have properties, and that they can be passed around by reference, makes them likely candidates for sharing data among scopes. Let's explore this.

Because object properties can be used to store data in memory as long as a variable referencing the object exists in memory, you can actually use them to get data into an event listener, and any changes to the data back out after an event handler executes. Consider this example.

const myButton = document.getElementById('my-button-id');
const someObject = {aProperty: 'Data'};

myButton.addEventListener('click', function() {
  console.log(someObject.aProperty);  // Expected Value: 'Data'

  someObject.aProperty = 'Data Again';  // Change the value
});

window.setInterval(function() {
  if (someObject.aProperty === 'Data Again') {
    console.log('Data Again: True');
    someObject.aProperty = 'Data';  // Reset value to wait for next event execution
  }
}, 5000);

In this example, even though the scope in which both the event listener and the interval function are defined would have finished executing before the original value of someObject.aProperty would have changed, because someObject persists in memory (by reference) in both the event listener and interval function, both have access to the same data (i.e. when one changes the data, the other can respond to the change).

const els = document.getElementsByTagName('*');

// Case 1
for(let i = 0; i < els.length; i++){
  els[i].addEventListener("click", function(e){/*do something*/}, false);
}

// Case 2
function processEvent(e){
  /* do something */
}

for(let i = 0 ; i < els.length; i++){
  els[i].addEventListener("click", processEvent, false);
}

In the first case above, a new (anonymous) handler function is created with each iteration of the loop. In the second case, the same previously declared function is used as an event handler, which results in smaller memory consumption because there is only one handler function created. Moreover, in the first case, it is not possible to call removeEventListener() because no reference to the anonymous function is kept (or here, not kept to any of the multiple anonymous functions the loop might create.) In the second case, it's possible to do myElement.removeEventListener("click", processEvent, false) because processEvent is the function reference.

Actually, regarding memory consumption, the lack of keeping a function reference is not the real issue; rather it is the lack of keeping a static function reference. In both problem-cases below, a function reference is kept, but it is redefined on each iteration. In the third case, the reference to the anonymous function is being reassigned with each iteration. In the fourth case, the entire function definition is unchanging, but it is still being repeatedly defined as if new. So neither is static. Therefore, though appearing to be multiple identical event listeners, in both cases each iteration will instead create a new listener with its own unique reference to the handler function.

const els = document.getElementsByTagName('*');

function processEvent(e){
  /* do something */
}

// For illustration only: Note the mistake of [j] for [i]. We are registering all event listeners to the first element

// Case 3
for(let i = 0, j = 0 ; i < els.length ; i++){
  els[j].addEventListener("click", processEvent = function(e){/* do something */}, false);
}

// Case 4
for(let i = 0, j = 0 ; i < els.length ; i++){
  function processEvent(e){/* do something */};
  els[j].addEventListener("click", processEvent, false);
}

Also in both case 3 and case 4, because the function reference was kept but repeatedly redefined with each addEventListener(), removeEventListener("click", processEvent, false) can still remove a listener, but now only the last one added.

Setting the passive option to true — as shown in the following example — enables performance optimizations that can dramatically improve the performance of an application.

/* Feature detection */
let passiveIfSupported = false;

try {
  window.addEventListener("test", null,
    Object.defineProperty(
      {},
      "passive",
      {
        get: function() { passiveIfSupported = { passive: true }; }
      }
    )
  );
} catch(err) {}

window.addEventListener('scroll', function(event) {
  /* do something */
  // can't use event.preventDefault();
}, passiveIfSupported );

The specification for addEventListener() defines the default value for the passive option as always being false. However, that introduces the potential for event listeners handling touch events and wheel events to block the browser's main thread while the browser is attempting to handle scrolling — possibly resulting in an enormous reduction in performance during scroll handling.

To prevent that problem, browsers other than Safari and Internet Explorer have changed the default value of the passive option to true for the wheel, mousewheel, touchstart and touchmove events on the document-level nodes Window, Document, and Document.body. That prevents the event listener from canceling the event, so it can't block page rendering while the user is scrolling.

Because of that, when you want to override that behavior and ensure the passive option is false in all browsers, you must explicitly set the option to false (rather than relying on the default).

You don't need to worry about the value of passive for the basic scroll event. Since it can't be canceled, event listeners can't block page rendering anyway.

In older browsers that don't support the options parameter to addEventListener(), attempting to use it prevents the use of the useCapture argument without proper use of feature detection.

• EventTarget.removeEventListener()
• Creating and triggering custom events
• More details on the use of this in event handlers