How to extend an existing JavaScript array with another array, without creating a new array

The .push method can take multiple arguments. You can use the spread operator to pass all the elements of the second array as arguments to .push:

>>> a.push(...b)

If your browser does not support ECMAScript 6, you can use .apply instead:

>>> a.push.apply(a, b)

Or perhaps, if you think it's clearer:

>>> Array.prototype.push.apply(a,b)

Please note that all these solutions will fail with a stack overflow error if array b is too long (trouble starts at about 100,000 elements, depending on the browser).
If you cannot guarantee that b is short enough, you should use a standard loop-based technique described in the other answer.

_{Update 2018: A better answer is a newer one of mine: a.push(...b). Don't upvote this one anymore, as it never really answered the question, but it was a 2015 hack around first-hit-on-Google :)}

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For those that simply searched for "JavaScript array extend" and got here, you can very well use Array.concat.

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

Concat will return a copy the new array, as thread starter didn't want. But you might not care (certainly for most kind of uses this will be fine).

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There's also some nice ECMAScript 6 sugar for this in the form of the spread operator:

const a = [1, 2, 3];
const b = [...a, 5, 4, 3];

(It also copies.)

You should use a loop-based technique. Other answers on this page that are based on using .apply can fail for large arrays.

A fairly terse loop-based implementation is:

Array.prototype.extend = function (other_array) {
    /* You should include a test to check whether other_array really is an array */
    other_array.forEach(function(v) {this.push(v)}, this);
}

You can then do the following:

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

DzinX's answer (using push.apply) and other .apply based methods fail when the array that we are appending is large (tests show that for me large is > 150,000 entries approx in Chrome, and > 500,000 entries in Firefox). You can see this error occurring in this jsperf.

An error occurs because the call stack size is exceeded when 'Function.prototype.apply' is called with a large array as the second argument. (MDN has a note on the dangers of exceeding call stack size using Function.prototype.apply - see the section titled "apply and built-in functions".)

For a speed comparison with other answers on this page, check out this jsperf (thanks to EaterOfCode). The loop-based implementation is similar in speed to using Array.push.apply, but tends to be a little slower than Array.slice.apply.

Interestingly, if the array you are appending is sparse, the forEach based method above can take advantage of the sparsity and outperform the .apply based methods; check out this jsperf if you want to test this for yourself.

By the way, do not be tempted (as I was!) to further shorten the forEach implementation to:

Array.prototype.extend = function (array) {
    array.forEach(this.push, this);
}

because this produces garbage results! Why? Because Array.prototype.forEach provides three arguments to the function it calls - these are: (element_value, element_index, source_array). All of these will be pushed onto your first array for every iteration of forEach if you use "forEach(this.push, this)"!