Comparison of Event Targets

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 5 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	nsIDOMNSEvent.idl	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	nsIDOMNSEvent.idl	The original target of the event, before any retargetings. See Anonymous Content#Event_Flow_and_Targeting for details.

Use of `explicitOriginalTarget` and `originalTarget`

TODO: Only available in a Mozilla-based browser? TODO: Only suitable for extension-developers?

範例

<!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 about dragenter and dragexit events.

範例

<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>

Comparison of Event Targets

Event targets

Use of explicitOriginalTarget and originalTarget

範例

Use of target and relatedTarget

範例

文件標籤與貢獻者

Use of `explicitOriginalTarget` and `originalTarget`

Use of `target` and `relatedTarget`