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• 句法
• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

Object.preventExtensions() method prevents new properties from ever being added to an object (i.e. prevents future extensions to the object).

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

句法

Object.preventExtensions(obj)
					

参数

obj

The object which should be made non-extensible.

返回值

The object being made non-extensible.

描述

An object is extensible if new properties can be added to it. Object.preventExtensions() marks an object as no longer extensible, so that it will never have properties beyond the ones it had at the time it was marked as non-extensible. Note that the properties of a non-extensible object, in general, may still be deleted . Attempting to add new properties to a non-extensible object will fail, either silently or by throwing a TypeError (most commonly, but not exclusively, when in 严格模式 ).

Object.preventExtensions() only prevents addition of own properties. Properties can still be added to the object prototype.

This method makes the [[prototype]] of the target immutable; any [[prototype]] re-assignment will throw a TypeError . This behavior is specific to the internal [[prototype]] property, other properties of the target object will remain mutable.

There is no way to make an object extensible again once it has been made non-extensible.

范例

Using Object.preventExtensions

// Object.preventExtensions returns the object
// being made non-extensible.
var obj = {};
var obj2 = Object.preventExtensions(obj);
obj === obj2; // true
// Objects are extensible by default.
var empty = {};
Object.isExtensible(empty); // === true
// ...but that can be changed.
Object.preventExtensions(empty);
Object.isExtensible(empty); // === false
// Object.defineProperty throws when adding
// a new property to a non-extensible object.
var nonExtensible = { removable: true };
Object.preventExtensions(nonExtensible);
Object.defineProperty(nonExtensible, 'new', {
  value: 8675309
}); // throws a TypeError
// In strict mode, attempting to add new properties
// to a non-extensible object throws a TypeError.
function fail() {
  'use strict';
  // throws a TypeError
  nonExtensible.newProperty = 'FAIL';
}
fail();
					

A non-extensible object's prototype is immutable:

var fixed = Object.preventExtensions({});
// throws a 'TypeError'.
fixed.__proto__ = { oh: 'hai' };
					

Non-object coercion

In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError . In ES2015, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return it.

Object.preventExtensions(1);
// TypeError: 1 is not an object (ES5 code)
Object.preventExtensions(1);
// 1                             (ES2015 code)
					

规范

规范
ECMAScript (ECMA-262) The definition of 'Object.preventExtensions' in that specification.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
preventExtensions	Chrome 6	Edge 12	Firefox 4	IE 9	Opera 12	Safari 5.1	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 12	Safari iOS 6	Samsung Internet Android 1.0	nodejs Yes
ES2015 behavior for non-object argument	Chrome 44	Edge 12	Firefox 35	IE 11	Opera 31	Safari 9	WebView Android 44	Chrome Android 44	Firefox Android 35	Opera Android 32	Safari iOS 9	Samsung Internet Android 4.0	nodejs Yes

图例

完整支持

完整支持

另请参阅

• Object.isExtensible()
• Object.seal()
• Object.isSealed()
• Object.freeze()
• Object.isFrozen()
• Reflect.preventExtensions()

元数据

• 最后修改： May 8, 2020

相关话题

1. 标准内置对象
2. Object
3. 特性
   1. Object.prototype.__proto__
   2. Object.prototype.constructor
4. 方法
   1. Object.assign()
   2. Object.create()
   3. Object.defineProperties()
   4. Object.defineProperty()
   5. Object.entries()
   6. Object.freeze()
   7. Object.fromEntries()
   8. Object.getOwnPropertyDescriptor()
   9. Object.getOwnPropertyDescriptors()
   10. Object.getOwnPropertyNames()
   11. Object.getOwnPropertySymbols()
   12. Object.getPrototypeOf()
   13. Object.is()
   14. Object.isExtensible()
   15. Object.isFrozen()
   16. Object.isSealed()
   17. Object.keys()
   18. Object.preventExtensions()
   19. Object.prototype.__defineGetter__()
   20. Object.prototype.__defineSetter__()
   21. Object.prototype.__lookupGetter__()
   22. Object.prototype.__lookupSetter__()
   23. Object.prototype.hasOwnProperty()
   24. Object.prototype.isPrototypeOf()
   25. Object.prototype.propertyIsEnumerable()
   26. Object.prototype.toLocaleString()
   27. Object.prototype.toSource()
   28. Object.prototype.toString()
   29. Object.prototype.valueOf()
   30. Object.seal()
   31. Object.setPrototypeOf()
   32. Object.values()
5. 继承：
6. Function
7. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
8. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()