Event.preventDefault()

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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>
</head>

<body>
    <p>Please click on the checkbox control.</p>
    <form>
        <label for="id-checkbox">Checkbox</label>
        <input type="checkbox" id="id-checkbox"/>
    </form>
    <script>
        document.querySelector("#id-checkbox").addEventListener("click", function(event){
            alert("preventDefault will stop you from checking this checkbox!")
            event.preventDefault();
        }, false);
    </script>
</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.

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