permutations

permu.js

'use strict';

const permU = (ar) => {
  if (ar.length <= 1) return [ar];
  const first = ar[0];
  const nPerm = permU(ar.slice(1));

  return nPerm.reduce((acc, value) => {
    for (let i=0; i <= value.length; ++i) {
      const start = value.slice(0, i);
      const tail = value.slice(i, value.length);
      const perm = start.concat(first).concat(tail);

      acc.push(perm);
    }
    return acc;
  }, []);
};

// Test Permuation:
const ALPH = 'abcdefghijklmnopqrstuvwxyz';
const flat = ar => ar.join('');

for (let i=1; i <= 5; ++i)
  console.log(`N ${i}: ${permU(ALPH.slice(0, i).split('')).map(flat)}`);