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).