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handler.preventExtensions()

JavaScript handler.preventExtensions()方法

handler.preventExtensions()方法用于捕获Object.preventExtensions方法。如果在对象上禁止扩展,则无法添加新属性。

语法

preventExtensions: function(target)

参数

target:目标对象。

返回值

返回布尔值。

浏览器支持

Chrome 49
Edge 12
Firefox 22
Opera 36

示例1

<script>
const proxy = new Proxy({}, {
  preventExtensions: function(target) {
   Object.preventExtensions(target);
    return !Object.isExtensible(target);
  }
});
document.writeln(Object.isExtensible(proxy));
//expected output:true
</script>
输出:
true

示例2

<script>
const proxy1 = new Proxy({}, {
  preventExtensions: function(target) {
    Object.preventExtensions(target);
    return !Object.isExtensible(target);
  }
});
document.writeln(Object.isExtensible(proxy1));
//expected output: true
document.writeln('<br/>');
document.writeln(Object.preventExtensions(proxy1));
// Object.preventExtensions prevent of object to access.
//expected output: [object Object]
document.writeln('<br/>');
document.writeln(Object.isExtensible(proxy1));
//expected output: false
</script>
输出:
true
[object Object]
false

示例3

<script>
var x = {
  first: false
};
var y ={
  preventExtensions(target) {
    target.canEvolve = false;
    Object.preventExtensions(target);
    return true;
  }
};
var proxy = new Proxy(x, y);
document.writeln(x.first);
// expected output: false
Object.preventExtensions(proxy);
document.writeln("<br/>");
document.writeln(x.first);
// expected output: false
</script>
输出:
false
false
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