flatten+flatMap.js

const log = console.log;

const isIterable = a => !!(a && a[Symbol.iterator]);

function *flatten(iter) {
  for (const a of iter) {
    if (isIterable(a)) for (const b of a) yield b;
    else yield a;
  }
}

function *map(f, iter) {
  for (const a of iter) yield f(a);
}

const flatMap = (f, iter) => flatten(map(f, iter));

log([...flatten([[1, 2], [3, 4]])]);
// [1, 2, 3, 4]

log([...flatMap(a => [a, a], [1, 2, 3])]);
// [1, 1, 2, 2, 3, 3]