如果我使用function.apply（undefined，args），为什么将this设置为window？

Question

Take a look at this example:

function aaa () {
    console.dir(this)
}

function bbb () {}

aaa.apply(undefined, [1,2,3]) // this in aaa is `window` object
aaa.apply(bbb, [1,2,3]) // this in aaa is `bbb` function

Why is this set to window in first apply case, even though I'm trying to force it to be undefined ?

Answer 1

When not in strict mode and either null or undefined is passed as the first argument to .apply() , this will be set to the global object which is window in a browser.

In strict mode, it would set this to the actual value you pass.

---

In general non-strict mode tried to be tolerant of mistakes and even "fix" some mistakes automatically for you. This proved to sometimes be a problem because things that should have been immediate coding errors were "covered up" by the system. strict mode was invented for a number of reasons and one of the reasons was to stop hiding coding errors.

Answer 2

Because that's how function calls work if you're not in strict mode . Without strict mode, a function's this reference always refers to an object, and it'll be the global object (ie window ) if no other object was given.