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

Question

There doesn't seem to be a way to extend an existing JavaScript array with another array, i.e. to emulate Python's extend method.

I want to achieve the following:

>>> a = [1, 2] [1, 2] >>> b = [3, 4, 5] [3, 4, 5] >>> SOMETHING HERE >>> a [1, 2, 3, 4, 5] 

I know there's a a.concat(b) method, but it creates a new array instead of simply extending the first one. I'd like an algorithm that works efficiently when a is significantly larger than b (i.e. one that does not copy a).

_{Note: This is not a duplicate of How to append something to an array? -- the goal here is to add the whole contents of one array to the other, and to do it "in place", i.e. without copying all elements of the extended array.}

Answer 1

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.