如何检测一个函数是否被称为构造函数？ [英] How to detect if a function is called as constructor?

问题描述

给定一个函数：

function x(arg) { return 30; }

您可以通过两种方式进行调用：

You can call it two ways:

result = x(4);
result = new x(4);

第一个返回30，第二个返回一个对象。

The first returns 30, the second returns an object.

如何检测函数在函数本身中被调用的方式？

How can you detect which way the function was called inside the function itself?

无论你的解决方案是什么，还有以下调用：

Whatever your solution is, it must work with the following invocation as well:

var Z = new x(); 
Z.lolol = x; 
Z.lolol();

所有解决方案目前认为 Z.lolol（）正在调用它作为构造函数。

All the solutions currently think the Z.lolol() is calling it as a constructor.

推荐答案

注意：现在可以在ES2015及更高版本。请参阅丹尼尔·韦纳的答案。

NOTE: This is now possible in ES2015 and later. See Daniel Weiner's answer.

我'想想你想要的是可能的[在ES2015之前]。

I don't think what you want is possible [prior to ES2015]. There simply isn't enough information available within the function to make a reliable inference.

查看ECMAScript第三版规范，所采取的步骤， new x（）本质上是：

Looking at the ECMAScript 3rd edition spec, the steps taken when new x() is called are essentially:

• 创建一个新对象


• 将其内部[[Prototype]]属性分配给 x
的原型属性

• 调用 x c>


• 如果调用 x 返回一个对象，返回它，否则返回新对象

• Create a new object
• Assign its internal [[Prototype]] property to the prototype property of x
• Call x as normal, passing it the new object as this
• If the call to x returned an object, return it, otherwise return the new object

被调用可用于执行代码，所以唯一可以在 x 里测试的是 this 值，这是这里所有的答案都在做。正如你所看到的，当调用 x 作为构造函数时，一个新的* x 实例，当调用 x 时， x 的现有实例传递为 this 除非，因为它是由 x 创建的每个新对象分配一个属性：

Nothing useful about how the function was called is made available to the executing code, so the only thing it's possible to test inside x is the this value, which is what all the answers here are doing. As you've observed, a new instance of* x when calling x as a constructor is indistinguishable from a pre-existing instance of x passed as this when calling x as a function, unless you assign a property to every new object created by x as it is constructed:

function x(y) {
    var isConstructor = false;
    if (this instanceof x // <- You could use arguments.callee instead of x here,
                          // except in in EcmaScript 5 strict mode.
            && !this.__previouslyConstructedByX) {
        isConstructor = true;
        this.__previouslyConstructedByX = true;
    }
    alert(isConstructor);
}

显然这不是理想的，因为你现在有一个额外的无用的属性由 x 构造的对象可以被覆盖，但我认为这是最好的。

Obviously this is not ideal, since you now have an extra useless property on every object constructed by x that could be overwritten, but I think it's the best you can do.

（*）实例是一个不准确的术语，但足够接近，并且比通过调用 x 作为构造函数创建的对象更简洁

(*) "instance of" is an inaccurate term but is close enough, and more concise than "object that has been created by calling x as a constructor"

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