Question

Why can I do this:

Array.prototype.foo = function() {
    this.splice(0, this.length);
    return this.concat([1,2,3]);
}

But I can’t do this:

Array.prototype.foo = function() {
    return this = [1,2,3];
}

Both functions destroy the value of this and change it to [1,2,3] but the second one throws the following error: Uncaught ReferenceError: Invalid left-hand side in assignment

I suspect it’s because allowing assignment means I could potentially change the array to something else (like a string), but I’m hoping someone out there knows for sure and/or has a more detailed explanation.

Answer 1

It’s not permitted to assign a value to this within a function. Suppose that you could do this, and your code looked something like:

Array.prototype.foo = function() {
    return this = [1, 2, 3];
}

var a = ["beans", "rice"];
a.foo();
// a now points to an object containing [1, 2, 3]

Now, what if you did this:

var a = ["beans", "rice"];
var b = a; // b refers to the same object as a
b.foo();
// what does b refer to now? how about a?

The act of calling a function .foo() on an object should not change the identity of the object. This would be very confusing for the caller if b suddenly started referring to a different object than a simply because some method was called.