Unable to understand useCapture parameter in addEventListener

312

votes

I have read article at https://developer.mozilla.org/en/DOM/element.addEventListener but unable to understand useCapture attribute. Definition there is:

If true, useCapture indicates that the user wishes to initiate capture. After initiating capture, all events of the specified type will be dispatched to the registered listener before being dispatched to any EventTargets beneath it in the DOM tree. Events which are bubbling upward through the tree will not trigger a listener designated to use capture.

In this code parent event triggers before child,so I am not able to understand its behavior.Document object has usecapture true and child div has usecapture set false and document usecapture is followed.So why document property is preferred over child.

function load() {
  document.addEventListener("click", function() {
    alert("parent event");
  }, true);

  document.getElementById("div1").addEventListener("click", function() {
    alert("child event");
  }, false);
}

<body onload="load()">
  <div id="div1">click me</div>
</body>

javascriptdomdom-events

371

votes

Events can be activated at two occasions: At the beginning ("capture"), and at the end ("bubble"). Events are executed in the order of how they're defined. Say, you define 4 event listeners:

window.addEventListener("click", function(){console.log(1)}, false);
window.addEventListener("click", function(){console.log(2)}, true);
window.addEventListener("click", function(){console.log(3)}, false);
window.addEventListener("click", function(){console.log(4)}, true);

The log messages will appear in this order:

2 (defined first, using capture=true)
4 (defined second using capture=true)
1 (first defined event with capture=false)
3 (second defined event with capture=false)

291

votes

I find this diagram is very useful for understanding the capture/target/bubble phases: http://www.w3.org/TR/2003/NOTE-DOM-Level-3-Events-20031107/events.html#Events-phases

Below, content extracted from the link.

Phases

The event is dispatched following a path from the root of the tree to this target node. It can then be handled locally at the target node level or from any target's ancestors higher in the tree. The event dispatching (also called event propagation) occurs in three phases and the following order:

The capture phase: the event is dispatched to the target's ancestors from the root of the tree to the direct parent of the target node.
The target phase: the event is dispatched to the target node.
The bubbling phase: the event is dispatched to the target's ancestors from the direct parent of the target node to the root of the tree.

graphical representation of an event dispatched in a DOM tree using the DOM event flow

The target's ancestors are determined before the initial dispatch of the event. If the target node is removed during the dispatching, or a target's ancestor is added or removed, the event propagation will always be based on the target node and the target's ancestors determined before the dispatch.

Some events may not necessarily accomplish the three phases of the DOM event flow, e.g. the event could only be defined for one or two phases. As an example, events defined in this specification will always accomplish the capture and target phases but some will not accomplish the bubbling phase ("bubbling events" versus "non-bubbling events", see also the Event.bubbles attribute).

88

votes

Capture Event (`useCapture = true`) vs Bubble Event (`useCapture = false`)

MDN Reference

Capture Event will be dispatch before Bubble Event
Event propagation order is
1. Parent Capture
2. Children Capture
3. Target Capture and Target Bubble
  - In the order they were registered
  - When the element is the target of the event, useCapture parameter doesn't matter (Thanks @bam and @legend80s)
4. Children Bubble
5. Parent Bubble
stopPropagation() will stop the flow

Demo

Result:

Parent Capture
Target Bubble 1

(Because Capture and Bubble of Target will trigger in the order they were registered, so Bubble event is trigger before Capture event)
Target Capture
Target Bubble 2
Parent Bubble

var parent = document.getElementById('parent'),
target = document.getElementById('target');

target.addEventListener('click', function (e) { 
console.log('Target Bubble 1');
// e.stopPropagation();
}, false);

target.addEventListener('click', function (e) { 
console.log('Target Capture');
// e.stopPropagation();
}, true);

target.addEventListener('click', function (e) { 
console.log('Target Bubble 2');
// e.stopPropagation();
}, false);

parent.addEventListener('click', function (e) { 
console.log('Parent Capture');
// e.stopPropagation();
}, true);

parent.addEventListener('click', function (e) { 
console.log('Parent Bubble');
// e.stopPropagation();
}, false);

<div id="parent">
    <button id="target" style="padding: 1em 0.8em;">
        Trigger event
    </button>
</div>

21

votes

When you say useCapture = true the Events execute top to down in the capture phase when false it does a bubble bottom to top.

13

votes

Summary:

The DOM spec described in:

https://www.w3.org/TR/2003/NOTE-DOM-Level-3-Events-20031107/events.html#Events-phases

works the following manner:

An event is dispatched following a path from the root (document) of the tree to the target node. The target node is the most deep HTML element, i.e. the event.target. The event dispatching (also called event propagation) occurs in three phases and the following order:

The capture phase: the event is dispatched to the target's ancestors from the root of the tree (document) to the direct parent of the target node.
The target phase: the event is dispatched to the target node. Target phase is always on the deepest html element on which the event was dispachted.
The bubbling phase: the event is dispatched to the target's ancestors from the direct parent of the target node to the root of the tree.

Example:

// bubbling handlers, third argument (useCapture) false (default)
document.getElementById('outerBubble').addEventListener('click', () => {
  console.log('outerBubble');
}, false)

document.getElementById('innerBubble').addEventListener('click', () => {
  console.log('innerBubble');
}, false)


// capturing handlers, third argument (useCapture)  true
document.getElementById('outerCapture').addEventListener('click', () => {
  console.log('outerCapture');
}, true)

document.getElementById('innerCapture').addEventListener('click', () => {
  console.log('innerCapture');
}, true)

div:hover{
  color: red;
  cursor: pointer;
}

<!-- event bubbling -->
<div id="outerBubble">
  <div id="innerBubble">click me to see Bubbling</div>
</div>


<!-- event capturing -->
<div id="outerCapture">
  <div id="innerCapture">click me to see Capturing</div>
</div>

The above example really illustrates the difference between event bubbling and event capturing. When adding the event listeners with addEventListener, there is a third element called useCapture. This a boolean which when set to true allows the event listener to use event capturing instead of event bubbling.

In our example when we set the useCapture argument to false we see that event bubbling takes place. First the event at the target phase is fired (logs innerBubble), and then via event bubbling the event in the parent element is fired (logs outerBubble).

When we set the useCapture argument to true we see that the event in the outer <div> is fired first. This is because the event is now fired in the capturing phase and not the bubbling phase.

12

votes

Code example:

<div id="div1" style="background:#9595FF">
  Outer Div<br />
  <div id="div2" style="background:#FFFFFF">
    Inner Div
  </div>
</div>

Javascript code:

d1 = document.getElementById("div1");
d2 = document.getElementById("div2");

if both are set to false

d1.addEventListener('click',function(){alert("Div 1")},false);
d2.addEventListener('click',function(){alert("Div 2")},false);

Executes: Onclicking Inner Div, alerts are displayed as: Div 2 > Div 1

Here the script is executed from the inner element: Event Bubbling (useCapture has been set to false)

div 1 is set to true and div 2 set to false

d1.addEventListener('click',function(){alert("Div 1")},true);
d2.addEventListener('click',function(){alert("Div 2")},false);

Executes: Onclicking Inner Div, alerts are displayed as: Div 1 > Div 2

Here the script is executed from the ancestor / outer element: Event Capturing (useCapture has been set to true)

div 1 is set to false and div 2 set to true

d1.addEventListener('click',function(){alert("Div 1")},false);
d2.addEventListener('click',function(){alert("Div 2")},true);

Executes: Onclicking Inner Div, alerts are displayed as: Div 2 > Div 1

Here the script is executed from the inner element: Event Bubbling (useCapture has been set to false)

div 1 is set to true and div 2 set to true

d1.addEventListener('click',function(){alert("Div 1")},true);
d2.addEventListener('click',function(){alert("Div 2")},true);

Executes: Onclicking Inner Div, alerts are displayed as: Div 1 > Div 2

Here the script is executed from the ancestor / outer element: Event Capturing since useCapture has been set to true

11

votes

It's all about event models: http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-flow You can catch event in bubbling phase or in capturing phase. Your choice.
Take a look at http://www.quirksmode.org/js/events_order.html - you'll find it very useful.

8

votes

Given the three phases of event travel:

The capture phase: the event is dispatched to the target's ancestors from the root of the tree to the direct parent of the target node.

The target phase: the event is dispatched to the target node.

The bubbling phase: the event is dispatched to the target's ancestors from the direct parent of the target node to the root of the tree.

useCapture indicates for which phases the event travel will be on:

If true, useCapture indicates that the user wishes to add the event listener for the capture phase only, i.e. this event listener will not be triggered during the target and bubbling phases. If false, the event listener will only be triggered during the target and bubbling phases

Source is the same as the second best answer: https://www.w3.org/TR/2003/NOTE-DOM-Level-3-Events-20031107/events.html#Events-phases

2

votes

The order of definition only matters if the items are at the same level. If you reverse the order of definition in your code you will get the same results.

However, if you reverse the useCapture setting on the two event handlers, the child event handler responds before that of the parent. The reason for this is that the child event handler will now be triggered in the capture phase which is prior to the bubbling phase in which the parent event handler will be triggered.

If you set useCapture to true for both event handlers--regardless of order of definition--the parent event handler will be triggered first because it comes before the child in the capturing phase.

Conversely, if you set useCapture to false for both event handlers--again regardless of order of definition--the child event handler will be triggered first because it comes before the parent in the bubbling phase.

Unable to understand useCapture parameter in addEventListener

9 Answers

Capture Event (useCapture = true) vs Bubble Event (useCapture = false)

Demo

Summary:

Example:

Capture Event (`useCapture = true`) vs Bubble Event (`useCapture = false`)