为什么我在JavaScript中需要`date`关键字作为'Date`的实例？ [英] Why do I need the `new` keyword for an instance of `Date` in JavaScript?

问题描述

我理解行为的区别。 Date（）返回表示当前日期的String， new Date（）返回Date对象的实例，我可以打电话的方法。

I understand the difference in behavior. Date() returns a String representing the current date, and new Date() returns an instance of the Date object whose methods I can call.

但我不知道为什么。 JavaScript是原型，因此 Date 是一个函数和一个对象，它具有也是对象的成员函数（方法）。但我没有编写或阅读任何行为方式的JavaScript，我想了解其中的区别。

But I don't know why. JavaScript is prototyped, so Date is a function and an object which has member functions (methods) which are also objects. But I haven't written or read any JavaScript that behaves this way, and I'd like to understand the difference.

有人可以给我看一些函数的示例代码有一个方法，用new运算符返回一个实例，并在直接调用时输出一个String？即如何发生这样的事情？

Can somebody show me some sample code of a function that has a method, returns an instance with the new operator, and outputs a String when called directly? i.e. how does something like this happen?

Date();                   // returns "Fri Aug 27 2010 12:45:39 GMT-0700 (PDT)"
new Date();               // returns Object
new Date().getFullYear(); // returns 2010
Date().getFullYear();     // throws exception!

非常具体的要求，我知道。我希望这是件好事。 ：）

Very specific request, I know. I hope that's a good thing. :)

推荐答案

大部分都可以自己做。根据ECMA规范，在没有 new 的情况下调用裸构造函数并获取字符串对于 Date 是特殊的，但是您可以模拟某些东西类似的。

Most of this is possible to do yourself. Calling the bare constructor without new and getting a string is special for Date per the ECMA spec, but you can simulate something similar for that.

这是你如何做到的。首先在构造函数模式中声明一个对象（例如，一个用 new 调用的函数，它返回这个 reference：

Here's how you'd do it. First declare an object in the constructor pattern (e.g. a function that is intended to be called with new and which returns its this reference:

var Thing = function() {
    // Check whether the scope is global (browser). If not, we're probably (?) being
    // called with "new". This is fragile, as we could be forcibly invoked with a 
    // scope that's neither via call/apply. "Date" object is special per ECMA script,
    // and this "dual" behavior is nonstandard now in JS. 
    if (this === window) { 
        return "Thing string";
    }

    // Add an instance method.
    this.instanceMethod = function() {
        alert("instance method called");
    }

    return this;
};

新事物可以在它们上面调用 instanceMethod（）。现在只需在Thing上添加一个静态函数：

New instances of Thing can have instanceMethod() called on them. Now just add a "static" function onto Thing itself:

Thing.staticMethod = function() {
    alert("static method called");
};

现在你可以这样做：

var t = new Thing(); 
t.instanceMethod();
// or...
new Thing().instanceMethod();
// and also this other one..
Thing.staticMethod();
// or just call it plain for the string:   
Thing();

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