EventTarget: addEventListener() method
Note:This feature is available inWeb Workers.
TheaddEventListener()
method of theEventTarget
interface
sets up a function that will be called whenever the specified event is delivered to the target.
Common targets areElement
,or its children,Document
,andWindow
,
but the target may be any object that supports events (such asIDBRequest
).
Note:TheaddEventListener()
method is therecommendedway to register an event listener. The benefits are as follows:
- It allows adding more than one handler for an event. This is particularly useful for libraries, JavaScript modules, or any other kind of code that needs to work well with other libraries or extensions.
- In contrast to using an
onXYZ
property, it gives you finer-grained control of the phase when the listener is activated (capturing vs. bubbling). - It works on any event target, not just HTML or SVG elements.
The methodaddEventListener()
works by adding a function, or an object that implements ahandleEvent()
function, to the list of event listeners for the specified event type
on theEventTarget
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.
Note:If a particular anonymous function is in the list of event listeners registered for a certain target, and then later in the code, an identical anonymous function is given in anaddEventListener
call, the second function willalsobe added to the list of event listeners for that target.
Indeed, anonymous functions are not identical even if defined using thesameunchanging source-code called repeatedly,even if in a loop.
Repeatedly defining the same unnamed function in such cases can be problematic. (SeeMemory issues,below.)
If an event listener is added to anEventTarget
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.
Syntax
addEventListener(type,listener)
addEventListener(type,listener,options)
addEventListener(type,listener,useCapture)
Parameters
type
-
A case-sensitive string representing theevent typeto 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 benull
,an object with ahandleEvent()
method, or a JavaScript function.See The event listener callbackfor details on the callback itself. options
Optional-
An object that specifies characteristics about the event listener. The available options are:
capture
Optional-
A boolean value indicating that events of this type will be dispatched to the registered
listener
before being dispatched to anyEventTarget
beneath it in the DOM tree. If not specified, defaults tofalse
. once
Optional-
A boolean value indicating that the
listener
should be invoked at most once after being added. Iftrue
,thelistener
would be automatically removed when invoked. If not specified, defaults tofalse
. passive
Optional-
A boolean value that, if
true
,indicates that the function specified bylistener
will never callpreventDefault()
.If a passive listener does callpreventDefault()
,the user agent will do nothing other than generate a console warning.If this option is not specified it defaults to
false
– except that in browsers other than Safari, it defaults totrue
forwheel
,mousewheel
,touchstart
andtouchmove
events. SeeUsing passive listenersto learn more. signal
Optional-
An
AbortSignal
.The listener will be removed when the givenAbortSignal
object'sabort()
method is called. If not specified, noAbortSignal
is associated with the listener.
useCapture
Optional-
A boolean value indicating whether events of this type will be dispatched to the registered
listener
beforebeing dispatched to anyEventTarget
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. SeeDOM Level 3 EventsandJavaScript Event orderfor a detailed explanation. If not specified,useCapture
defaults tofalse
.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 thecapturingphase are called before event listeners in any non-capturing phases.
wantsUntrusted
Optional Non-standard-
A Firefox (Gecko)-specific parameter. If
true
,the listener receives synthetic events dispatched by web content (the default isfalse
for browserchromeandtrue
for regular web pages). This parameter is useful for code found in add-ons, as well as the browser itself.
Return value
None (undefined
).
Usage notes
The event listener callback
The event listener can be specified as either a callback function or
an object whosehandleEvent()
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 onEvent
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:
functionhandleEvent(event){
if(event.type==="fullscreenchange"){
/* handle a full screen toggle */
}else{
/* handle a full screen toggle error */
}
}
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 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 thepassive
option:
letpassiveSupported=false;
try{
constoptions={
getpassive(){
// This function will be called when the browser
// attempts to access the passive property.
passiveSupported=true;
returnfalse;
},
};
window.addEventListener("test",null,options);
window.removeEventListener("test",null,options);
}catch(err){
passiveSupported=false;
}
This creates anoptions
object with a getter function for the
passive
property; the getter sets a flag,
passiveSupported
,totrue
if it gets called. That
means that if the browser checks the value of thepassive
property on the
options
object,passiveSupported
will be
set totrue
;otherwise, it will remainfalse
.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 callremoveEventListener()
to clean up after
ourselves. (Note thathandleEvent()
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 themouseup
event on the elementsomeElement
.For the third parameter, if
passiveSupported
istrue
,we're specifying an
options
object withpassive
set to
true
;otherwise, we know that we need to pass a Boolean, and we pass
false
as the value of theuseCapture
parameter.
You can learn more in theImplementing feature detectiondocumentation and the explainer about
EventListenerOptions
from theWeb Incubator Community Group.
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.
When attaching a handler function to an element usingaddEventListener()
,
the value ofthis
inside the handler will be a reference to
the element. It will be the same as the value of thecurrentTarget
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 ownthis
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 ofthis
in a manner consistent with theaddEventListener()
;an occurrence ofthis
within the code represents a reference to the element.
<tableid="my_table"onclick="console.log(this.id);">
<!-- `this` refers to the table; logs 'my_table' -->
…
</table>
Note that the value ofthis
inside a function,called bythe code
in the attribute value, behaves as perstandard rules.This is
shown in the following example:
<script>
functionlogID(){
console.log(this.id);
}
</script>
<tableid="my_table"onclick="logID();">
<!-- when called, `this` will refer to the global object -->
…
</table>
The value ofthis
withinlogID()
is a reference to the global
objectWindow
(orundefined
in the case ofstrict mode.
Specifying "this" using bind()
TheFunction.prototype.bind()
method lets you establish a fixed
this
context for all subsequent calls — bypassing problems where it's unclear whatthis
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 withoutbind()
:
classSomething{
name="Something Good";
constructor(element){
// 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
}
onclick1(event){
console.log(this.name);// undefined, as `this` is the element
}
onclick2(event){
console.log(this.name);// 'Something Good', as `this` is bound to the Something instance
}
}
consts=newSomething(document.body);
Another solution is using a special function calledhandleEvent()
to catch
any events:
classSomething{
name="Something Good";
constructor(element){
// Note that the listeners in this case are `this`, not this.handleEvent
element.addEventListener("click",this,false);
element.addEventListener("dblclick",this,false);
}
handleEvent(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;
}
}
}
consts=newSomething(document.body);
Another way of handling the reference tothis
is to use an arrow function, which doesn't create a separatethis
context.
classSomeClass{
name="Something Good";
register(){
window.addEventListener("keydown",(e)=>{
this.someMethod(e);
});
}
someMethod(e){
console.log(this.name);
switch(e.code){
case"ArrowUp":
// some code here…
break;
case"ArrowDown":
// some code here…
break;
}
}
}
constmyObject=newSomeClass();
myObject.register();
Getting data into and out of an event listener
Event listeners only take one argument,
anEvent
or a subclass ofEvent
,
which is automatically passed to the listener, and the return value is ignored.
Therefore, to get data into and out of an event listener, instead of passing the data through parameters and return values, you need to createclosuresinstead.
The functions passed as event listeners have access to all variables declared in the outer scopes that contain the function.
constmyButton=document.getElementById("my-button-id");
letsomeString="Data";
myButton.addEventListener("click",()=>{
console.log(someString);
// 'Data' on first click,
// 'Data Again' on second click
someString="Data Again";
});
console.log(someString);// Expected Value: 'Data' (will never output 'Data Again')
Readthe function guidefor more information about function scopes.
Memory issues
constelts=document.getElementsByTagName("*");
// Case 1
for(consteltofelts){
elt.addEventListener(
"click",
(e)=>{
// Do something
},
false,
);
}
// Case 2
functionprocessEvent(e){
// Do something
}
for(consteltofelts){
elt.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)
becauseprocessEvent
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 astaticfunction reference.
Using passive listeners
If an event has a default action — for example, awheel
event that scrolls the container by default — the browser is in general unable to start the default action until the event listener has finished, because it doesn't know in advance whether the event listener might cancel the default action by callingEvent.preventDefault()
.If the event listener takes too long to execute, this can cause a noticeable delay, also known asjank,before the default action can be executed.
By setting thepassive
option totrue
,an event listener declares that it will not cancel the default action, so the browser can start the default action immediately, without waiting for the listener to finish. If the listener does then callEvent.preventDefault()
,this will have no effect.
The specification foraddEventListener()
defines the default value for thepassive
option as always beingfalse
.However, to realize the scroll performance benefits of passive listeners in legacy code, modern browsers have changed the default value of thepassive
option totrue
for thewheel
,mousewheel
,touchstart
andtouchmove
events on the document-level nodesWindow
,Document
,andDocument.body
.That prevents the event listener fromcanceling 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 thepassive
option isfalse
,you must explicitly set the option tofalse
(rather than relying on the default).
You don't need to worry about the value ofpassive
for the basicscroll
event.
Since it can't be canceled, event listeners can't block page rendering anyway.
SeeImproving scroll performance using passive listenersfor an example showing the effect of passive listeners.
Older browsers
In older browsers that don't support theoptions
parameter to
addEventListener()
,attempting to use it prevents the use of the
useCapture
argument without proper use offeature detection.
Examples
Add a simple listener
This example demonstrates how to useaddEventListener()
to watch for mouse
clicks on an element.
HTML
<tableid="outside">
<tr>
<tdid="t1">one</td>
</tr>
<tr>
<tdid="t2">two</td>
</tr>
</table>
JavaScript
// Function to change the content of t2
functionmodifyText(){
constt2=document.getElementById("t2");
constisNodeThree=t2.firstChild.nodeValue==="three";
t2.firstChild.nodeValue=isNodeThree?"two":"three";
}
// Add event listener to table
constel=document.getElementById("outside");
el.addEventListener("click",modifyText,false);
In this code,modifyText()
is a listener forclick
events
registered usingaddEventListener()
.A click anywhere in the table bubbles
up to the handler and runsmodifyText()
.
Result
Add an abortable listener
This example demonstrates how to add anaddEventListener()
that can be aborted with anAbortSignal
.
HTML
<tableid="outside">
<tr>
<tdid="t1">one</td>
</tr>
<tr>
<tdid="t2">two</td>
</tr>
</table>
JavaScript
// Add an abortable event listener to table
constcontroller=newAbortController();
constel=document.getElementById("outside");
el.addEventListener("click",modifyText,{signal:controller.signal});
// Function to change the content of t2
functionmodifyText(){
constt2=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 callabort()
from theAbortController
we passed to theaddEventListener()
call. That results in the value remaining as "three" forever because we no longer have any code listening for a click event.
Result
Event listener with anonymous function
Here, we'll take a look at how to use an anonymous function to pass parameters into the event listener.
HTML
<tableid="outside">
<tr>
<tdid="t1">one</td>
</tr>
<tr>
<tdid="t2">two</td>
</tr>
</table>
JavaScript
// Function to change the content of t2
functionmodifyText(new_text){
constt2=document.getElementById("t2");
t2.firstChild.nodeValue=new_text;
}
// Function to add event listener to table
constel=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 themodifyText()
function, which is
responsible for actually responding to the event.
Result
Event listener with an arrow function
This example demonstrates a simple event listener implemented using arrow function notation.
HTML
<tableid="outside">
<tr>
<tdid="t1">one</td>
</tr>
<tr>
<tdid="t2">two</td>
</tr>
</table>
JavaScript
// Function to change the content of t2
functionmodifyText(new_text){
constt2=document.getElementById("t2");
t2.firstChild.nodeValue=new_text;
}
// Add event listener to table with an arrow function
constel=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 ownthis
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.
Example of options usage
HTML
<divclass="outer">
outer, once & none-once
<divclass="middle"target="_blank">
middle, capture & none-capture
<aclass="inner1"href="https:// mozilla.org"target="_blank">
inner1, passive & preventDefault(which is not allowed)
</a>
<aclass="inner2"href="https://developer.mozilla.org/"target="_blank">
inner2, none-passive & preventDefault(not open new page)
</a>
</div>
</div>
<hr/>
<buttonclass="clear-button">Clear logs</button>
<sectionclass="demo-logs"></section>
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
constouter=document.querySelector(".outer");
constmiddle=document.querySelector(".middle");
constinner1=document.querySelector(".inner1");
constinner2=document.querySelector(".inner2");
constcapture={
capture:true,
};
constnoneCapture={
capture:false,
};
constonce={
once:true,
};
constnoneOnce={
once:false,
};
constpassive={
passive:true,
};
constnonePassive={
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);
functiononceHandler(event){
log("outer, once");
}
functionnoneOnceHandler(event){
log("outer, none-once, default\n");
}
functioncaptureHandler(event){
//event.stopImmediatePropagation();
log("middle, capture");
}
functionnoneCaptureHandler(event){
log("middle, none-capture, default");
}
functionpassiveHandler(event){
// Unable to preventDefault inside passive event listener invocation.
event.preventDefault();
log("inner1, passive, open new page");
}
functionnonePassiveHandler(event){
event.preventDefault();
//event.stopPropagation();
log("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 theoptions
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. SeeSafely detecting option supportfor details.
Event listener with multiple options
You can set more than one of the options in theoptions
parameter. In the following example we are setting two options:
passive
,to assert that the handler will not callpreventDefault()
once
,to ensure that the event handler will only be called once.
HTML
<buttonid="example-button">You have not clicked this button.</button>
<buttonid="reset-button">Click this button to reset the first button.</button>
JavaScript
constbuttonToBeClicked=document.getElementById("example-button");
constresetButton=document.getElementById("reset-button");
// the text that the button is initialized with
constinitialText=buttonToBeClicked.textContent;
// the text that the button contains after being clicked
constclickedText="You have clicked this button.";
// we hoist the event listener callback function
// to prevent having duplicate listeners attached
functioneventListener(){
buttonToBeClicked.textContent=clickedText;
}
functionaddListener(){
buttonToBeClicked.addEventListener("click",eventListener,{
passive:true,
once:true,
});
}
// when the reset button is clicked, the example button is reset,
// and allowed to have its state updated again
resetButton.addEventListener("click",()=>{
buttonToBeClicked.textContent=initialText;
addListener();
});
addListener();
Result
Improving scroll performance using passive listeners
The following example shows the effect of settingpassive
.It includes a<div>
that contains some text, and a check box.
HTML
<divid="container">
<p>
But down there it would be dark now, and not the lovely lighted aquarium she
imagined it to be during the daylight hours, eddying with schools of tiny,
delicate animals floating and dancing slowly to their own serene currents
and creating the look of a living painting. That was wrong, in any case. The
ocean was different from an aquarium, which was an artificial environment.
The ocean was a world. And a world is not art. Dorothy thought about the
living things that moved in that world: large, ruthless and hungry. Like us
up here.
</p>
</div>
<div>
<inputtype="checkbox"id="passive"name="passive"checked/>
<labelfor="passive">passive</label>
</div>
JavaScript
The code adds a listener to the container'swheel
event, which by default scrolls the container. The listener runs a long-running operation. Initially the listener is added with thepassive
option, and whenever the checkbox is toggled, the code toggles thepassive
option.
constpassive=document.querySelector("#passive");
passive.addEventListener("change",(event)=>{
container.removeEventListener("wheel",wheelHandler);
container.addEventListener("wheel",wheelHandler,{
passive:passive.checked,
once:true,
});
});
constcontainer=document.querySelector("#container");
container.addEventListener("wheel",wheelHandler,{
passive:true,
once:true,
});
functionwheelHandler(){
functionisPrime(n){
for(letc=2;c<=Math.sqrt(n);++c){
if(n%c===0){
returnfalse;
}
}
returntrue;
}
constquota=1000000;
constprimes=[];
constmaximum=1000000;
while(primes.length<quota){
constcandidate=Math.floor(Math.random()*(maximum+1));
if(isPrime(candidate)){
primes.push(candidate);
}
}
console.log(primes);
}
Result
The effect is that:
- Initially, the listener is passive, so trying to scroll the container with the wheel is immediate.
- If you uncheck "passive" and try to scroll the container using the wheel, then there is a noticeable delay before the container scrolls, because the browser has to wait for the long-running listener to finish.
Specifications
Specification |
---|
DOM Standard #ref-for-dom-eventtarget-addeventlistener③ |
Browser compatibility
BCD tables only load in the browser