ES6 functions for calculating factorials and finding all possible combinations of an array

utils.js

export function factorial(num) {
  if (num < 0) return -1;
  else if (num === 0) return 1;
  return num * factorial(num - 1);
}

export function allPossibleCombos(list, size) {
  if (size > list.length) return [];
  const results = [];

  const generate = (combo, index, count) => {
    const isFullSet = count === 1;

    for (index; index < list.length; index++) {
      const next = [...combo, list[index]];

      if (isFullSet) {
        results.push(next);
      } else {
        generate(next, index + 1, count - 1);
      }
    }
  };

  generate([], 0, size);
  return results;
}

utils.spec.js

import { factorial, allPossibleCombos } from "./utils.js";

describe("factorial", () => {
  test("returns the product of all positive integers less than or equal to a given positive integer", () => {
    const integers = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1];
    const expected = [3628800, 362880, 40320, 5040, 720, 120, 24, 6, 2, 1];
    for (let i = 0; i < integers.length; i++) {
      const actual = factorial(integers[i]);
      expect(actual).toBe(expected[i]);
    }
  });
});

describe("allPossibleCombos", () => {
  test("Generates a list of all possible combinations of items in an array", () => {
    const list = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"];
    const size = 5;
    const result = allPossibleCombos(list, size);

    const possibilities =
      factorial(list.length) /
      (factorial(size) * factorial(list.length - size));

    expect(result.length).toBe(possibilities);
  });
});