esclip快捷键构造函数,如何检测一个函数是否被称为构造函数？

Given a function:

function x(arg) { return 30; }

You can call it two ways:

result = x(4);

result = new x(4);

The first returns 30, the second returns an object.

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

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

var Z = new x();

Z.lolol = x;

Z.lolol();

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

解决方案

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

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.

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

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

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 //

// except in in EcmaScript 5 strict mode.

&& !this.__previouslyConstructedByX) {

isConstructor = true;

this.__previouslyConstructedByX = true;

}

alert(isConstructor);

}

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.

(*) "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"