从三元运算符返回的函数中丢失上下文 [英] Losing context from function returned by ternary operator

问题描述

我知道如何解决这个特定的问题，但是我想知道为什么会发生.基本上，当我尝试调用这样的函数时:

I know how to get around this particular problem, but I would like to know why it happens. Basically, when I try to invoke a function like this:

(callFoo ? this.foo : this.bar)();

它调用正确的foo函数，但是在foo内部， this 是全局的窗口对象，而不是我期望的对象.

It calls the correct foo function, but inside of foo, this is the global, window object instead of the object I expect.

我希望这可以做同样的事情，但事实并非如此:

I would expect that this does the same thing but it does not:

(this.foo)();

上面的代码调用正确的函数并维护正确的上下文( this 是我期望的).

The code above calls the right function and maintains the correct context (this is what I expect it to be).

这是一个jsfiddle，供您试用.

有人可以解释发生了什么吗?我知道如何解决这个问题(我什至都​​不喜欢这种语法)，但是我仍然想知道如果您从三元运算符返回一个函数，为什么 this 成为窗口.

Could someone please explain what is going on? I understand how to get around the problem (I'm not even a fan of that syntax), but I still want to know why this becomes the window if you return a function from a ternary operator.

编辑
我想完善我的问题:对我来说，这很有意义:

EDIT
I'd like to refine my question: It makes sense to me that this:

(callFoo ? this.foo : this.bar)();

等效于:

var f = (callFoo ? this.foo : this.bar);
f();

这对我来说很有意义，为什么 this 成为该函数中的窗口.

And it makes sense to me why this becomes the window within that function.

为什么这里不发生相同的事情:

Why doesn't the same thing happen here:

(this.foo)();

推荐答案

造成差异的原因:

var obj = new (function MyConstructor(){
    this.getConstructor = function(){ return this.constructor.name; }
});

任何操作员对操作数进行操作时，结果的工作方式都非常类似于函数的返回值.传递的对象方法不再像绑定到对象一样对待.

When operands are operated on by any operator, the results work much like the return value of a function. An object method passed is no longer treated like it's tied to the object.

(function(){ return obj.getConstructor; })(); //'Window'

但是，在除属性访问以外的其他任何内容都没有发生的情况下，paren只会被忽略，而不是被当作运算符本身.所以:

But in the case where nothing is happening inside the parens other than property access, the parens are simply ignored rather than treated as operators themselves. So:

(obj.getConstructor)(); //'MyConstructor'

真的等同于:

obj.getConstructor();

但是添加导致该方法的任何有效操作

But add a valid operation of any kind resulting in the method:

(false || obj.getConstructor)(); //'Window'

并且obj.getConstructor被视为已传递的方法，而不是通过'.'绑定到对象的方法.关联.

And obj.getConstructor is treated as a method that's been passed rather than a method tied to an object via the '.' association.

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