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Comparison of Event Targets - Web APIs 编辑

Event targets

It's easy to get confused about which target to examine when writing an event handler. This article should clarify the use of the target properties.

There are five targets to consider:

Property	Defined in	Purpose
`event.target`	DOM Event Interface	The DOM element on the lefthand side of the call that triggered this event, eg: element.dispatchEvent(event)
`event.currentTarget`	DOM Event Interface	The `EventTarget` whose `EventListeners` are currently being processed. As the event capturing and bubbling occurs, this value changes.
`event.relatedTarget`	DOM MouseEvent Interface	Identifies a secondary target for the event.
`event.explicitOriginalTarget`	Event.webidl	If the event was retargeted for some reason other than an anonymous boundary crossing, this will be set to the target before the retargeting occurs. For example, mouse events are retargeted to their parent node when they happen over text nodes (bug 185889), and in that case `.target` will show the parent and `.explicitOriginalTarget` will show the text node. Unlike `.originalTarget`, `.explicitOriginalTarget` will never contain anonymous content.
`event.originalTarget`	Event.webidl	The original target of the event, before any retargetings. See Anonymous Content#Event_Flow_and_Targeting for details.
event.composedTarget	Event.webidl	The original non-native target of the event before composition from Shadow DOM.

Use of explicitOriginalTarget and originalTarget

TODO: Only available in a Mozilla-based browser?

TODO: Only suitable for extension-developers?

Examples

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>Comparison of Event Targets</title>
    <style>
        table {
            border-collapse: collapse;
            height: 150px;
            width: 100%;
        }
        td {
            border: 1px solid #ccc;
            font-weight: bold;
            padding: 5px;
            min-height: 30px;
        }
        .standard {
            background-color: #99ff99;
        }
        .non-standard {
            background-color: #902D37;
        }
    </style>
</head>
<body>
    <table>
    <thead>
        <tr>
            <td class="standard">Original target dispatching the event <small>event.target</small></td>
            <td class="standard">Target who's event listener is being processed <small>event.currentTarget</small></td>
            <td class="standard">Identify other element (if any) involved in the event <small>event.relatedTarget</small></td>
            <td class="non-standard">If there was a retargetting of the event for some reason <small> event.explicitOriginalTarget</small> contains the target before retargetting (never contains anonymous targets)</td>
            <td class="non-standard">If there was a retargetting of the event for some reason <small> event.originalTarget</small> contains the target before retargetting (may contain anonymous targets)</td>
        </tr>
    </thead>
    <tr>
        <td id="target"></td>
        <td id="currentTarget"></td>
        <td id="relatedTarget"></td>
        <td id="explicitOriginalTarget"></td>
        <td id="originalTarget"></td>
    </tr>
</table>
<p>Clicking on the text will show the difference between explicitOriginalTarget, originalTarget, and target</p>
<script>
    function handleClicks(e) {
        document.getElementById('target').innerHTML = e.target;
        document.getElementById('currentTarget').innerHTML = e.currentTarget;
        document.getElementById('relatedTarget').innerHTML = e.relatedTarget;
        document.getElementById('explicitOriginalTarget').innerHTML = e.explicitOriginalTarget;
        document.getElementById('originalTarget').innerHTML = e.originalTarget;
    }

    function handleMouseover(e) {
        document.getElementById('target').innerHTML = e.target;
        document.getElementById('relatedTarget').innerHTML = e.relatedTarget;
    }

    document.addEventListener('click', handleClicks, false);
    document.addEventListener('mouseover', handleMouseover, false);
</script>
</body>
</html>

Use of target and relatedTarget

The relatedTarget property for the mouseover event holds the node that the mouse was previously over. For the mouseout event, it holds the node that the mouse moved to.

Event type	event.target	event.relatedTarget
`mouseover`	the EventTarget which the pointing device entered	the EventTarget which the pointing device exited
`mouseout`	the EventTarget which the pointing device exited	the EventTarget which the pointing device entered

TODO: Also needs descriptions for dragenter and dragexit events.

Example

<hbox id="outer">
  <hbox id="inner"
        onmouseover="dump('mouseover ' + event.relatedTarget.id + ' > ' + event.target.id + '\n');"
        onmouseout="dump('mouseout  ' + event.target.id + ' > ' + event.relatedTarget.id + '\n');"
        style="margin: 100px; border: 10px solid black; width: 100px; height: 100px;" />
</hbox>

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