Calculates all permutations for a given array of elements following Heap's Algorithm

permutations.js

// Calculates all permutations for a given array of elements following Heap's Algorithm

// how to use it...
console.log(generatePermutations([1, 2]));
console.log(generatePermutations([1, 2, 3]));
console.log(generatePermutations([1, 2, 3, 4, 5, 6, 7, 8, 9]));

function generatePermutations(inputArray) {
    let output = [];

    generate(inputArray.length, inputArray);

    return output;

    function generate(elementsNumber, array) {
        if (elementsNumber === 1) {
            return output.push(array.slice(0, array.length));
        } else {
            for (let i = 0; i < elementsNumber - 1; i += 1) {
                generate(elementsNumber - 1, array);
                if (elementsNumber % 2 === 0) {
                    swap(i, elementsNumber - 1, array);
                } else {
                    swap(0, elementsNumber - 1, array);
                }
            }
            generate(elementsNumber - 1, array);
        }
    }

    function swap(x, y, array) {
        let z = array[x];
        array[x] = array[y];
        array[y] = z;
    }
}