function flat(x, n, fm, ft) // x: a nested iterable // n: maximum depth (-1 ⇒ all) // fm: map function (v, i, x) // ft: flatten test (v, i, x) [isList]

const xiterable = require('extra-iterable'); var x = [[1, 2], [3, [4, [5]]]]; [...xiterable.flat(x)]; // → [1, 2, 3, 4, 5] [...xiterable.flat(x, 1)]; // → [1, 2, 3, [4, [5]]] [...xiterable.flat(x, 2)]; // → [1, 2, 3, 4, [5]]