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

One option is to use reduce:

let arrs = [[1, 2], [3, 4], [5, 6]];
arrs.reduce((a, b) => [...a, ...b], []);

Of course, this is a slow solution (quadratic time). Alternatively, if you can use Lodash, _.flatten does exactly what you want, and does it more efficiently (linear time).

EDIT

Or, adapted from Xotic750's comment below,

[].concat(...arrs);

Which should be efficient (linear time).