Solution to https://www.credit-suisse.com/pwp/hr/en/codeitsuisse/index.html?WT.i_short-url=%2Fcodeitsuisse&WT.i_target-url=https%3A%2F%2Fwww.credit-suisse.com%2Fpwp%2Fhr%2Fen%2Fcodeitsuisse%2Findex.html

shuffle.js

function shuffle (action, inputString, optionalArgs) {
  if (action && !parseInt(action)) throw 'invalid action type';
  if (action < 1 || action > 4) throw 'action out of range';
  if (action == 1) {
    const end = inputString.slice(-1 * optionalArgs);
    return end + inputString.slice(0, -1 * optionalArgs);
  } else if (action == 2) {
    return inputString.split('').reverse().join('');
  } else if (action == 3) {
    var n = 0; var x = '';
    let chars = inputString.split('');
    chars.forEach((e) => {
      const m = inputString.match(new RegExp(e, 'g')).length;
      if (m > n) { x = e; n = m; }
      inputString.replace(e, '');
    });
    return x;
  } else if (action == 4) {
    let ordering = optionalArgs.split('').reduce((a, c, i) => {a[c] = i; return a;}, {});
    return inputString.split('').sort((a, b) => {
      return ordering[a] - ordering[b];
    }).join('');
  }
}