Flattens nested array to given depth.
This is part of package extra-array.
array.flat(x, [n], [fm], [ft]);
// x: a nested array
// n: maximum depth (-1 => all)
// fm: map function (v, i, x)
// ft: test function (v, i, x)
const array = require("extra-array");
var x = [[1, 2], [3, [4, [5]]]];
array.flat(x);
// [ 1, 2, 3, 4, 5 ]
array.flat(x, 1);
// [ 1, 2, 3, [ 4, [ 5 ] ] ]
array.flat(x, 2);
// [ 1, 2, 3, 4, [ 5 ] ]