A straightforward solution for "No Repeats Please" coding interview exercise at freecodecamp.org (https://www.freecodecamp.org/learn/coding-interview-prep/algorithms/no-repeats-please)

no-repeats-please.js

// A straightforward solution for "No Repeats Please" 
// coding interview exercise at freecodecamp.org 
// (https://www.freecodecamp.org/learn/coding-interview-prep/algorithms/no-repeats-please)
//
// Luca Parolari
//
// GitHub: github.com/lparolari
// Email: luca.parolari23@gmail.com
// Telegram: @lparolari

function genPerm(str) {
  /* base case */
  if (str.length == 1) return [str];

  /* inductive case */
  let res = []

  for (let i = 0; i < str.length; i++) {
    const a = str[i]

    const substr = str.substring(0, i) + str.substring(i + 1, str.length)

    const perms = genPerm(substr)

    for (const perm of perms) {
      res.push(a + perm)
    }
  }

  return res;
}

function hasRepeatedConsecutiveLetters(str) {
  for (let i = 0; i < str.length - 1; i++) {
    if (str[i] === str[i + 1]) {
      return true;
    }
  }
  return false;
}

function permAlone(str) {
  const perms = genPerm(str)

  let counter = 0

  for (const perm of perms) {
    if (!hasRepeatedConsecutiveLetters(perm)) {
      counter ++;
    }
  }

  return counter;
}

permAlone('aab');  // 2