On: ‘This’ in JavaScript

[eiriklv]

Simple(r) explanation; this is whatever is "left" of the function call (i.e - obj.method() => this === obj inside the called method), unless you've used .bind/call/apply which explicitly sets what this should be via an argument.

If you're doing a direct function call like myFunc() there is nothing "left" of the function call. But you might think of it as actually doing window.myFunc(), which again explains why this === window. In strict mode this behavior has been "fixed" so that this === undefined.

It's all about how a function is called.

[Cakez0r → eiriklv]

What about setTimeout(obj.method, 1)?

(Spoilers: this == window, not what is left of the function call)

[nhpatt → Cakez0r]

You are not calling the function, you are passing a reference to setTimeout that calls it directly, without the obj. before the call.

@getify explains it flawlessly, there are 4 modes: new, obj., bind/apply/call and default (window or error in strict mode)

[eiriklv → Cakez0r]

As nhpatt notes you're not actually calling obj.method here, you're passing it as an argument, which will be called at a later time.

Looking at a possible implementation of setTimeout makes it clearer:

  function setTimeout(func, delay) {
    // wait for the delay to complete..
    func();
  }

Here you can see that when the function is called there's nothing to the left of it.

[Cakez0r → eiriklv]

Yup, I get it. I was just pointing out that there are some edge cases to the "context is whatever is left of the function" rule-of-thumb.