Question

Does anybody know? Couldn t find this question asked before, even though it seems fairly basic.

Answer 1

The context (the this keyword) it s not completely implicit, it can be set and changed explicitly.

For example:

function test () {
  alert(this);
}

test.call("Hello world");

The test function is called with a string as the context.

So in conclusion, you cannot know what this is unless you explicitly define it, or you are inside the function.

Answer 2

The same function will see different values of this depending on how it called. See Crockford for details, but there are four cases:

Invoked as a simple function, it is bound to the global/window object.
Invoked as a method on an object, it refers to that object.
Invoked as a constructor via the new keyword, it is the newly instantiated object, which inherits from the object stored in function s own prototype property.
Invoked by its own apply or call method, it is the first argument supplied.

If these cases sound complex, tedious, and error-prone, all the more reason to avoid relying on this outside of methods, where it makes the most sense anyway.

友情链接