Using es6 spread to concat multiple arrays

Another option could be:

const nArrays = [
  [1, 2, 3, 4, 5],
  [6, 7, 8, 9],
  [10, 11]
];
const flattened = [].concat(...nArrays);
console.log(flattened)

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).