permutations.js

/**
 * Input: 1, 2, 3, 4
 * Output: 
 *   1
 *   1, 2
 *   1, 2, 3
 *   1, 2, 3, 4
 *   1, 2, 4
 *   1, 3
 *   1, 3, 4
 *   1, 4
 */
function permutations(input, index = 0, prefix = [], stack = []) {
    let next = [ ...prefix, input[index] ];
    stack.push(next);
    for (let i = index + 1; i < input.length; i++) {
        permutations(input, i, next, stack);
    }
    return stack;
}