EventTarget.addEventListener()

The EventTarget.addEventListener() method adds the specified EventListener-compatible object to the list of event listeners for the specified event type on the EventTarget on which it is called. The event target may be an Element in a document, the Document itself, a Window, or any other object that supports events (such as XMLHttpRequest).

Syntax

target.addEventListener(type, listener[, options]);
target.addEventListener(type, listener[, useCapture]);
target.addEventListener(type, listener[, useCapture, wantsUntrusted  ]); // Gecko/Mozilla only

Parameters

type

A string representing the event type to listen for.

listener

The object which 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.

options Optional

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

• capture: A Boolean 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.  
• once: A Boolean indicating that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked.
• passive: A Boolean indicating that the listener will never call preventDefault(). If it does, the user agent should ignore it and generate a console warning. See Improving scrolling performance with passive listeners to learn more.
• mozSystemGroup: A Boolean indicating that the listener should be added to the system group. Available only in code running in XBL or in Firefox's chrome.

useCapture Optional

A Boolean 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. 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 which 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. Events in the target phase will trigger all listeners on an element in the order they were registered, regardless of the useCapture parameter.

Note: useCapture has not always been optional. Ideally, you should include it for the widest possible browser compatibility.

wantsUntrusted

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

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.

Safely detecting option support

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 which 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:

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

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.

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.

Example

Add a simple 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() {
  var t2 = document.getElementById("t2");
  if (t2.firstChild.nodeValue == "three") {
    t2.firstChild.nodeValue = "two";
  } else {
    t2.firstChild.nodeValue = "three";
  }
}
// add event listener to table
var el = document.getElementById("outside");
el.addEventListener("click", modifyText, false);

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

If you want to pass parameters to the listener function, you may use an anonymous function.

Event listener with anonymous function

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) {
  var t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;    
}
// Function to add event listener to table
var el = document.getElementById("outside");
el.addEventListener("click", function(){modifyText("four")}, false);

Notes

Why use addEventListener?

addEventListener is the way to register an event listener as specified in W3C DOM. The benefits are as follows:

• It allows adding more than a single handler for an event. This is particularly useful for DHTML libraries or Mozilla extensions that need to work well with other libraries/extensions.
• It gives you finer-grained control of the phase when the listener is activated (capturing vs. bubbling).
• It works on any DOM element, not just HTML elements.

The alternative, older way to register event listeners, is described below.

Adding a listener during event dispatch

If an EventListener is added to an EventTarget while it is processing an event, that event does not trigger the listener. However, that same listener may be triggered during a later stage of event flow, such as the bubbling phase.

Multiple identical event listeners

If multiple identical EventListeners are registered on the same EventTarget with the same parameters, the duplicate instances are discarded. They do not cause the EventListener to be called twice, and they do not need to be removed manually with the removeEventListener method.

The value of this within the handler

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.

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

If an event attribute (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 which 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. 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:

<table id="t" onclick="modifyText();">
  . . .

The value of this within modifyText(), when called via the onclick event, is a reference to the global (window) object (or undefined in the case of strict mode).

This is an example with and without bind:

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

A problem in the example above is that you cannot remove the listener with bind. Another solution is using a special function called handleEvent to catch any events:

var 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);
}
var 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 which contains the fields that need to be accessed:

class SomeClass {
  constructor() {
    this.name = 'Something Good';
  }
  register() {
    var that = this;
    window.addEventListener('keydown', function(e) {return that.someMethod(e);});
  }
  someMethod(e) {
    console.log(this.name);
    switch(e.keyCode) {
      case 5:
        // some code here...
        break;
      case 6:
        // some code here...
        break;
    }
  }
}
var myObject = new SomeClass();
myObject.register();

Legacy Internet Explorer and attachEvent

In Internet Explorer versions before IE 9, you have to use attachEvent, rather than the standard addEventListener. For IE, we modify the preceding example to:

if (el.addEventListener) {
  el.addEventListener('click', modifyText, false); 
} else if (el.attachEvent)  {
  el.attachEvent('onclick', modifyText);
}

There is a drawback to attachEvent: The value of this will be a reference to the window object, instead of the element on which it was fired.

Compatibility

You can work around addEventListener(), removeEventListener(), Event.preventDefault(), and Event.stopPropagation() not being supported by IE 8 by using the following code at the beginning of your script. The code supports the use of handleEvent and also the DOMContentLoaded event.

(function() {
  if (!Event.prototype.preventDefault) {
    Event.prototype.preventDefault=function() {
      this.returnValue=false;
    };
  }
  if (!Event.prototype.stopPropagation) {
    Event.prototype.stopPropagation=function() {
      this.cancelBubble=true;
    };
  }
  if (!Element.prototype.addEventListener) {
    var eventListeners=[];
    var addEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var self=this;
      var wrapper=function(e) {
        e.target=e.srcElement;
        e.currentTarget=self;
        if (typeof listener.handleEvent != 'undefined') {
          listener.handleEvent(e);
        } else {
          listener.call(self,e);
        }
      };
      if (type=="DOMContentLoaded") {
        var wrapper2=function(e) {
          if (document.readyState=="complete") {
            wrapper(e);
          }
        };
        document.attachEvent("onreadystatechange",wrapper2);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper2});
        if (document.readyState=="complete") {
          var e=new Event();
          e.srcElement=window;
          wrapper2(e);
        }
      } else {
        this.attachEvent("on"+type,wrapper);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper});
      }
    };
    var removeEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var counter=0;
      while (counter<eventListeners.length) {
        var eventListener=eventListeners[counter];
        if (eventListener.object==this && eventListener.type==type && eventListener.listener==listener) {
          if (type=="DOMContentLoaded") {
            this.detachEvent("onreadystatechange",eventListener.wrapper);
          } else {
            this.detachEvent("on"+type,eventListener.wrapper);
          }
          eventListeners.splice(counter, 1);
          break;
        }
        ++counter;
      }
    };
    Element.prototype.addEventListener=addEventListener;
    Element.prototype.removeEventListener=removeEventListener;
    if (HTMLDocument) {
      HTMLDocument.prototype.addEventListener=addEventListener;
      HTMLDocument.prototype.removeEventListener=removeEventListener;
    }
    if (Window) {
      Window.prototype.addEventListener=addEventListener;
      Window.prototype.removeEventListener=removeEventListener;
    }
  }
})();

Older way to register event listeners

addEventListener() was introduced with the DOM 2 Events specification. Before then, event listeners were registered as follows:

// Passing a function reference — do not add '()' after it, which would call the function!
el.onclick = modifyText;
// Using a function expression
element.onclick = function() {
  // ... function logic ...
};

This method replaces the existing click event listener(s) on the element if there are any. Other events and associated event handlers such as blur (onblur) and keypress (onkeypress) behave similarly.

Because it was essentially part of DOM 0, this technique for adding event listeners is very widely supported and requires no special cross–browser code. It is normally used to register event listeners dynamically unless the extra features of addEventListener() are needed.

Memory issues

var i;
var els = document.getElementsByTagName('*');
// Case 1
for(i=0 ; i<els.length ; i++){
  els[i].addEventListener("click", function(e){/*do something*/}, false);
}
// Case 2
function processEvent(e){
  /*do something*/
}
for(i=0 ; i<els.length ; i++){
  els[i].addEventListener("click", processEvent, false);
}

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

Improving scrolling performance with passive listeners

The default value for the passive option is false. Starting in Chrome 56 (desktop, Chrome for Android, and Android webview) the default value for touchstart and touchmove is true and calls to preventDefault() are not permitted. To override this behavior, you set the passive option to false as shown in the example below. This change prevents the listener from blocking page rendering while a user is scrolling. A demo is available on the Google Developer site. Please note that Edge does not support the options argument at all, and adding it will prevent the use of the useCapture argument.

var elem = document.getElementById('elem');
elem.addEventListener('touchmove', function listener() {
  /* do something */
}, { passive: false });

Setting passive isn't important for the basic scroll event, as it cannot be canceled, so its listener can't block page rendering anyway.

Specifications

Browser compatibility

[1] Although WebKit has explicitly added [optional] to the useCapture parameter as recently as June 2011, it had been working before the change. The new change landed in Safari 5.1 and Chrome 13.

[2] Before Chrome 49, the type and listener parameters were optional.

[3] Before Firefox 6, the browser would throw an error if the useCapture parameter was not explicitly false. Before Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6), addEventListener() would throw an exception if the listener parameter was null; now the method returns without error, but without doing anything.

[4] Older versions of Internet Explorer support the proprietary EventTarget.attachEvent method instead.

[5] For backward compatibility, browsers that support options allow the third parameter to be either options or Boolean.

See also

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