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概要
如果事件可以被取消，就取消事件（即取消事件的預設行為）。但不會影響事件的傳遞，事件仍會繼續傳遞。
語法
event.preventDefault();
範例
Toggling a checkbox is the default action of clicking on a checkbox. This example demonstrates how to prevent that from happening:
<!DOCTYPE html>
<html>
<head>
<title>preventDefault example</title>
<script>
function stopDefAction(evt) {
evt.preventDefault();
}
document.getElementById('my-checkbox').addEventListener(
'click', stopDefAction, false
);
</script>
</head>
<body>
<p>Please click on the checkbox control.</p>
<form>
<input type="checkbox" id="my-checkbox" />
<label for="my-checkbox">Checkbox</label>
</form>
</body>
</html>
You can see preventDefault in action here.
The following example demonstrates how invalid text input can be stopped from reaching the input field with preventDefault().
<!DOCTYPE html> <html> <head> <title>preventDefault example</title> <script>
function Init () {
var myTextbox = document.getElementById('my-textbox');
myTextbox.addEventListener( 'keypress', checkName, false );
}
function checkName(evt) {
var charCode = evt.charCode;
if (charCode != 0) {
if (charCode < 97 || charCode > 122) {
evt.preventDefault();
alert(
"Please use lowercase letters only."
+ "\n" + "charCode: " + charCode + "\n"
);
}
}
}
</script>
</head>
<body onload="Init ()">
<p>Please enter your name using lowercase letters only.</p>
<form>
<input type="text" id="my-textbox" />
</form>
</body>
</html>
Here is the result of the preceding code:
備註
Calling preventDefault during any stage of event flow cancels the event, meaning that any default action normally taken by the implementation as a result of the event will not occur.
Note: As of Gecko 6.0, calling preventDefault() causes the event.defaultPrevented property's value to become true.
你可以查看 Event.cancelable 屬性來檢查事件是否能夠被取消。對一個不能被取消的事件呼叫 preventDefault() 方法是沒有任何效果的。
preventDefault() 方法不會停止事件傳遞。若要停止事件繼續傳遞，可以使用 Event.stopPropagation() 方法。
規範
| Specification | Status | Comment |
|---|---|---|
| DOM The definition of 'Event.preventDefault()' in that specification. |
Living Standard | |
| DOM4 The definition of 'Event.preventDefault()' in that specification. |
Recommendation | |
| Document Object Model (DOM) Level 2 Events Specification The definition of 'Event.preventDefault()' in that specification. |
Recommendation | Initial definition. |