Object.prototype.defineGetter()

¥Syntax

__defineGetter__(prop, func)

¥Parameters

prop

包含 getter func 绑定到的属性名称的字符串。

func

绑定到指定属性的查找的函数。

¥Return value

无 (undefined)。

¥None (undefined).

¥Exceptions

TypeError

如果 func 不是函数，则抛出该异常。

¥Description

所有从 Object.prototype 继承的对象（即除 null-原型对象 之外的所有对象）都继承 __defineGetter__() 方法。此方法允许在预先存在的对象上定义 getter。这相当于 Object.defineProperty(obj, prop, { get: func, configurable: true, enumerable: true })，这意味着该属性是可枚举和可配置的，并且任何现有的 setter（如果存在）都会被保留。

¥All objects that inherit from Object.prototype (that is, all except null-prototype objects) inherit the __defineGetter__() method. This method allows a getter to be defined on a pre-existing object. This is equivalent to Object.defineProperty(obj, prop, { get: func, configurable: true, enumerable: true }), which means the property is enumerable and configurable, and any existing setter, if present, is preserved.

__defineGetter__() 在规范中定义为 "规范性 可选性"，这意味着不需要实现即可实现此目的。然而，所有主流浏览器都实现了它，并且由于它的持续使用，它不太可能被删除。如果浏览器实现了 __defineGetter__()，它还需要实现 __lookupGetter__()、__lookupSetter__() 和 __defineSetter__() 方法。

¥__defineGetter__() is defined in the spec as "normative optional", which means no implementation is required to implement this. However, all major browsers implement it, and due to its continued usage, it's unlikely to be removed. If a browser implements __defineGetter__(), it also needs to implement the __lookupGetter__(), __lookupSetter__(), and __defineSetter__() methods.

¥Examples

¥Using __defineGetter__()

const o = {};
o.__defineGetter__("gimmeFive", function () {
  return 5;
});
console.log(o.gimmeFive); // 5

¥Defining a getter property in standard ways

你可以使用 get 语法在对象首次初始化时定义 getter。

¥You can use the get syntax to define a getter when the object is first initialized.

const o = {
  get gimmeFive() {
    return 5;
  },
};
console.log(o.gimmeFive); // 5

你还可以在创建对象后使用 Object.defineProperty() 在对象上定义 getter。与 __defineGetter__() 相比，此方法允许你控制 getter 的可枚举性和可配置性，以及定义 symbol 属性。Object.defineProperty() 方法也适用于 null-原型对象，null-原型对象 不是从 Object.prototype 继承的，因此没有 __defineGetter__() 方法。

¥You may also use Object.defineProperty() to define a getter on an object after it's been created. Compared to __defineGetter__(), this method allows you to control the getter's enumerability and configurability, as well as defining symbol properties. The Object.defineProperty() method also works with null-prototype objects, which don't inherit from Object.prototype and therefore don't have the __defineGetter__() method.

const o = {};
Object.defineProperty(o, "gimmeFive", {
  get() {
    return 5;
  },
  configurable: true,
  enumerable: true,
});
console.log(o.gimmeFive); // 5

¥See also

• core-js 中 Object.prototype.__defineGetter__ 的 Polyfill
• Object.prototype.__defineSetter__()
• get
• Object.defineProperty()
• Object.prototype.__lookupGetter__()
• Object.prototype.__lookupSetter__()
• JS 指南：定义 Getter 和 Setter
• 火狐浏览器错误 647423