unfold

Builds an array, using an iterator function and an initial seed value.

Use a while loop and Array.prototype.push() to call the function repeatedly until it returns false. The iterator function accepts one argument (seed) and must always return an array with two elements ([value, nextSeed]) or false to terminate.

const unfold = (fn, seed) => {
  let result = [],
    val = [null, seed];
  while ((val = fn(val[1]))) result.push(val[0]);
  return result;
};

var f = (n) => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]

function unfold<T>(fn: (i: T) => false | T[], seed: T): T[] {
  const result: T[] = [];
  let val: false | T[] = [null, seed];
  while ((val = fn(val[1]))) result.push(val[0]);
  return result;
}

const f = (n: number) => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]