javascript How Array.prototype.push concatenates

Question

I've seen the Array's push method used to replace concatenation, but I'm not entirely sure how it works.

var a = [1,2,3];
var b = [4,5,6];
Array.prototype.push.apply(a,b);

How does this concatenate in place rather than returning a new array?

Answer 1

.apply() takes two arguments:

fun.apply(thisArg[, argsArray])

You're passing a as your this object and b as your argument list, so your code is really calling .push() with the elements of b as your arguments:

var a = [1, 2, 3];
a.push(4, 5, 6);

Now, .push() just mutates your original array.

Answer 2

Using this. To try to describe it see this customPush function.

function customPush() {
    var i = 0,
        len = arguments.length;

    for(; i < len; i++) {
        this[this.length] = arguments[i];
    }
};


var a = [1,2,3];
var b = [4,5,6];
customPush.apply(a,b);