When to use 'Array.prototype' and when to use 'this' in JavaScript?

Question

Let's take this example from The Good Parts book:

Array.method('unshift', function () {
    this.splice.apply(this,[0,0].concat(Array.prototype.slice.apply(arguments)));
    return this;
});

Why did the author use this.splice in one place and Array.prototype.slice in other?

I tried swapping out this and Array.prototype with each other and got errors like the following:

TypeError: Cannot read property 'slice' of undefined

but I am still not sure about, how to know when to should use this or Array.prototype.

Answer 1

In the first call, this refers to the array on which unshift was called, and so it inherits splice from Array.prototype.

In the second call, though, the code uses slice on something that isn't an array (the arguments pseudo-array, which doesn't have a slice method). So in that case, Crockford accessed the method via Array.prototype.

Technically, he could have used this.slice in the second location, like this:

Array.method('unshift', function () {
    this.splice.apply(this,[0,0].concat(this.slice.apply(arguments)));
    return this;
});

...but it probably would have been misleading, since the second call has nothing to do with the current array referenced by this.