javascript-如何获取对象以返回不是对象本身的值 [英] javascript - how to get an object to return a value that is not the object itself

问题描述

创建x = new Date()对象时，如果将其放入console.log(x)，则会输出字符串.

When creating a x = new Date() object, if you put it into console.log(x) a string will be output.

有没有一种方法可以使自定义对象返回一个本身不是对象的值

Is there a way to make a custom object that will return a different value that is not an object itself

推荐答案

基本上两个方法返回的值都不是对象本身.

Basically two methods returns values, which are not the object itself.

Object#toString

每个对象都有一个toString()方法，当该对象将被表示为文本值或以一种期望的方式引用一个对象时，该方法会自动被调用.默认情况下，toString()方法由Object派生的每个对象继承.如果自定义对象中未覆盖此方法，则toString()返回"[object type]"，其中 type 是对象类型.

Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected. By default, the toString() method is inherited by every object descended from Object. If this method is not overridden in a custom object, `toString() returns "[object type]", where type is the object type.

Object#valueOf

JavaScript调用valueOf方法将对象转换为原始值.您很少需要自己调用valueOf方法.当遇到需要原始值的对象时，JavaScript会自动调用它.

JavaScript calls the valueOf method to convert an object to a primitive value. You rarely need to invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.

默认情况下，valueOf方法由 Object .每个内置核心对象都重写此方法以返回适当的值.如果对象没有原始值，则valueOf返回对象本身.

By default, the valueOf method is inherited by every object descended from Object. Every built-in core object overrides this method to return an appropriate value. If an object has no primitive value, valueOf returns the object itself.

您可以在自己的代码中使用valueOf将内置对象转换为原始值.创建自定义对象时，可以覆盖Object.prototype.valueOf()来调用自定义方法，而不是默认的Object方法.

You can use valueOf within your own code to convert a built-in object into a primitive value. When you create a custom object, you can override Object.prototype.valueOf() to call a custom method instead of the default Object method.

要针对您的问题进行分解，可以将这两种方法都实现为自定义函数.

To break it down for your question, you can implement both methods to a custom function.

当两种方法都实现并返回原始值时，首先调用valueOf.如果valueOf返回一个对象，则会调用toString方法.

When both methods are implemented and return primitive values, the valueOf is called first. If valueOf returns an object, toString method is called.

您可能会看到以下文章:伪造的运算符重载于JavaScript

You might have a look to this article: Fake operator overloading in JavaScript

function Custom(value) {
    this.value = value;
}

Custom.prototype.valueOf = function () {
    console.log('valueOf');
    return this.value * 5;
};


var custom = new Custom(7);

console.log('' + custom); // no toString

Custom.prototype.toString = function () {
    console.log('toString');
    return this.value * 3;
};

Custom.prototype.valueOf = function () {
    console.log('valueOf');
    return {};
};

console.log('' + custom);

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