how can I concatenate two arrays in javascript?

Question

I saw this response for extending one array with another so I tried:

console.log(['a', 'b'].push.apply(['c', 'd']));

but it prints:

2

shouldn't it print:

['a', 'b', 'c', 'd']

if not what was i doing wrong?

Answer 1

if not what was i doing wrong?

First of all, .push returns the new length of the array:

var arr = [1, 1, 1];
console.log(arr.push(1)); // 4
console.log(arr); // [1, 1, 1, 1]

Second, .apply needs two arguments: The object you want to apply the function to, and an array of arguments. You only pass a single argument, so your code is basically equivalent to:

['c', 'd'].push()

I.e. you are not adding anything to the ['c', 'd'] array. That also explains why you see 2 in the output: ['c', 'd'] has length 2 and .push() doesn't add any elements to it so it still has length 2.

If you want to use .push to mutate the original array (instead of creating a new one like .concat does), it would have to look like:

var arr = ['a', 'b'];
arr.push.apply(arr, ['c', 'd']); // equivalent to Array.prototype.push.apply(arr, [...])
//              ^   ^--------^
//    apply to arr   arguments
console.log(arr); // ['a', 'b', 'c', 'd']

---

See also

• How to append something to an array?
• Javascript push array values into another array