Why “this” refers to Window in forEach in javascript? [duplicate]

Answer 1

.forEach() specifies the value of this within the iterator based on its 2nd parameter, thisArg.

arr.forEach(callback[, thisArg])

So, it will only use a particular object if you provide it:

arr.forEach(function(e){
    console.log(this);
}, arr); // <---

Otherwise, the value of this will be the default value of a normal function call -- either undefined in strict mode or the global object (window in browsers) in non-strict.

function foo(e) {
    console.log(this);
}

foo();            // [object Window]

[1].forEach(foo); // (same)

---

Though, the arr is still provided to the iterator, just as its 3rd argument:

arr.forEach(function (e, i, arr) {
    console.log(arr);
});

Answer 2

this gets defined using by new, or the object invoking this function, right?

Yes. And in this case, it's forEach that invokes your function. Since no special precautions^[1] are made, it simply passes nothing for this^[2] just like any non-method function call, which will result into the global object as your function is sloppy code^[3].

_{1: Like passing a second argument to forEach, which would be used for this
2: The spec says it should pass undefined
3: Meaning, that it does not use strict mode. In strict code, this would literally really be undefined.}