How to concat arrays in JS in mutating way? [duplicate]

Question

What is the best way to concat arrays in mutating way in JS? My problem:

var a = [1,2,3]
var b = a;
a = a.concat([4,5,6]) //<-- typical way of array concatenation in JS
console.log(a) //[1,2,3,4,5,6]
console.log(b) //[1,2,3] <-- I'd like it to be the same like a here

I can use some loop of course, but I wonder if there is some faster and cleaner way to achieve it.

Answer 1

push mutates an array, but it expects sequence of arguments, so we use apply:

var a = [1,2,3]
var b = a
a.push.apply(a, [4,5,6])
console.log(a) //=> [1, 2, 3, 4, 5, 6]
console.log(b) //=> [1, 2, 3, 4, 5, 6]

In ES6 you could use spread operator ...:

a.push(...[4, 5, 6])