[goldenkey]

I couldn't help but revise the code because it failed to even use minus-equals and is severely unoptimized.

Here is my rendition. Mind you, this is mainly for IE as ES6 has a String.padStart method. I would also use Array(n).fill() but again, IE does not support it, so manual string appending is the sacrifice to make..

  module.exports = function(str, len, ch) {
    if (ch === undefined || ch === '') ch = ' ';

    str += "";
    len -= str.length;
    len /= ch.length;

    var pre = "";
    while (--len >= 0) pre += ch;

    return pre + str;
  }