Using es6 spread to concat multiple arrays

Question

We all know you can do:

let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arr3 = [...arr1, ...arr2]; // [1,2,3,3,4,5]

But how do you make this dynami

Answer 1

You could use a recursive function and Array.prototype.concat

const concatN = (x,...xs) =>
  x === undefined ? [] : x.concat(concatN(...xs))

console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]

You can do the same thing using reduce and Array.prototype.concat. This is similar to the accepted answer but doesn't senselessly use spread syntax where x.concat(y) is perfectly acceptable (and likely heaps faster) in this case

const concatN = (...xs) =>
  xs.reduce((x,y) => x.concat(y), [])

console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]