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ch17_14_Methods_of_All_Objects.md

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14. Methods of All Objects 所有对象的方法

Almost all objects have Object.prototype in their prototype chain:

几乎所有对象的原型链上都有 Object.prototype:

> Object.prototype.isPrototypeOf({})
true
> Object.prototype.isPrototypeOf([])
true
> Object.prototype.isPrototypeOf(/xyz/)
true

The following subsections describe the methods that Object.prototypeprovides for its prototypees.

以下小节将讲述Object.prototype 提供给它的实例的方法：

14.1 Conversion to Primitive 转换为原始值

The following two methods are used to convert an object to a primitive value:

以下两种方法用于将对象转换成原始值：

  • Object.prototype.toString()

    Returns a string representation of an object:

    返回对象的字符串形式：

> ({ first: 'John', last: 'Doe' }.toString())
'[object Object]'
> [ 'a', 'b', 'c' ].toString()
'a,b,c'`

  • Object.prototype.valueOf()

    This is the preferred way of converting an object to a number. The default implementation returns this:

    将对象转换成数值优先使用的方法。默认返回this（即对象本身）:

> var obj = {};
> obj.valueOf() === obj
> true

valueOf is overridden by wrapper constructors to return the wrapped primitive:

包装函数覆盖了valueOf() 方法，返回被包装的原始值：

> new Number(7).valueOf()
7

The conversion to number and string (whether implicit or explicit) builds on the conversion to primitive (for details, see Algorithm: ToPrimitive()—Converting a Value to a Primitive). That is why you can use the aforementioned two methods to configure those conversions. valueOf() is preferred by the conversion to number:

无论是隐式地还是显式地转换成数值或字符串，都是基于成原始值的转换（详细内容，见算法：ToPrimitive() -- 将值转换为原始值 ）。这就是为什么你可以使用前面提到的两种方法来设置这些转换。转换为数值时，优先使用valueOf() ：

3 * { valueOf: function () { return 5 } }
15

toString() is preferred by the conversion to string:

转换为字符串时，优先使用toString() :

> String({ toString: function () { return 'ME' } })
'Result: ME'

The conversion to boolean is not configurable; objects are always considered to be true (see Converting to Boolean).

转换为布尔值是不可配置的；对象总是转换为true （见 转换成布尔值）.

14.2 Object.prototype.toLocaleString()

This method returns a locale-specific string representation of an object. The default implementation calls toString(). Most engines don’t go beyond this support for this method. However, the ECMAScript Internationalization API (see The ECMAScript Internationalization API), which is supported by many modern engines, overrides it for several built-in constructors.

此方法返回一个对象的指定地区的字符串表示，默认调用toString() 方法。大部分的 JS 引擎对这个方法的支持仅此而已。然而，ECMAScript 国际化 API （见 The ECMAScript Internationalization API）被许多现代引擎所支持，为几个内置的构造函数覆盖了这个方法。

14.3 Prototypal Inheritance and Properties 原型继承 和 属性

The following methods help with prototypal inheritance and properties:

以下方法是用于原型继承和属性的：

  • Object.prototype.isPrototypeOf(obj)

    Returns true if the receiver is part of the prototype chain of obj:

    如果接收者（调用该方法的对象）在obj 的 原型链上，返回true：

> var proto = { };
> var obj = Object.create(proto);
> proto.isPrototypeOf(obj)
true
> obj.isPrototypeOf(obj)
false

  • Object.prototype.hasOwnProperty(key)

    Returns true if this owns a property whose key is key. “Own” means that the property exists in the object itself and not in one of its prototypes.

    如果this （调用该方法的对象）自身拥有键名为key 的属性，返回true 。“Own” 意味着这个属性存在对象本身，而不是在对象的原型上。

​ WARNING 警告 You normally should invoke this method generically (not directly), especially on objects whose properties you don’t know statically. Why and how is explained in Iteration and Detection of Properties:

通常，你需要调用通用方法（而不要直接调用这个方法），尤其是当你不知道这个对象的静态属性时。在 属性的遍历及检测 已解释过为什么及如何调用通用方法：

> var proto = { foo: 'abc' };
> var obj = Object.create(proto);
> obj.bar = 'def';
> Object.prototype.hasOwnProperty.call(obj, 'foo')
false
> Object.prototype.hasOwnProperty.call(obj, 'bar')
true

  • Object.prototype.propertyIsEnumerable(propKey)

    Returns true if the receiver has a property with the key propKey that is enumerable and false otherwise:

    如果接收者拥有键名为propKey 的属性，且该属性是可枚举的，返回true ，否则返回 false ：

> var obj = { foo: 'abc' };
> obj.propertyIsEnumerable('foo')
true
> obj.propertyIsEnumerable('toString')
false
> obj.propertyIsEnumerable('unknown')
false