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

General Discussions
1

I want to extend an existing JavaScript array with another array (emulating Python’s extend method).

I know that concat() creates a new array, but I’m looking for a way to modify the original array directly. The goal is to avoid copying the entire array if one is significantly larger than the other. Any efficient solution?

2

Hey! One of the most straightforward ways is using push() along with the spread operator. So if you have:

let a = [1, 2];
let b = [3, 4, 5];
a.push(...b);

That’ll update a in place. Just a heads-up though m if b is really large, spreading it might hit the call stack limit. So it works great for small to moderate arrays, but not ideal if b has, say, 100,000+ items.

3

If you’re worried about performance with large arrays, you can go old-school and loop through the second array:

let a = [1, 2];
let b = [3, 4, 5];
for (let i = 0; i < b.length; i++) {
  a.push(b[i]);
}

It’s simple, doesn’t risk stack issues, and it modifies a directly. Works well when you’re handling large datasets.

4

Another neat option is using forEach() — a bit more readable than a for loop for some:

let a = [1, 2];
let b = [3, 4, 5];
b.forEach(item => a.push(item));

It still updates the original array a and avoids creating a new one. This can be especially nice if you want to insert additional logic while pushing items too.