index.js

/* This program is free software. It comes without any warranty, to
     * the extent permitted by applicable law. You can redistribute it
     * and/or modify it under the terms of the Do What The Fuck You Want
     * To Public License, Version 2, as published by Sam Hocevar. See
     * http://www.wtfpl.net/ for more details. */
'use strict';
module.exports = leftPad;

var cache = [
  '',
  ' ',
  '  ',
  '   ',
  '    ',
  '     ',
  '      ',
  '       ',
  '        ',
  '         '
];

function leftPad (str, len, ch) {
  // convert `str` to a `string`
  str = str + '';

  // `len` is the `pad`'s length now
  len = len - str.length;

  console.log('DEBUG: str: ' + str + ', len: ' + len, ', ch: ' + ch);

  // doesn't need to pad
  if (len <= 0) {
    return str;
  }

  // `ch` defaults to `' '`
  if (!ch && ch !== 0){
    ch = ' ';
  }

  // convert `ch` to a `string` cuz it could be a number
  ch = ch + '';
  // cache common use cases
  if (ch === ' ' && len < 10){
    return cache[len] + str;
  }

  // `pad` starts with an empty string
  var pad = '';
  // loop
  while (true) {
    // add `ch` to `pad` if `len` is odd
    if (len & 1) {
      pad += ch;
    }
    // divide `len` by 2, ditch the remainder
    len >>= 1;
    // "double" the `ch` so this operation count grows logarithmically on `len`
    // each time `ch` is "doubled", the `len` would need to be "doubled" too
    // similar to finding a value in binary search tree, hence O(log(n))
    console.log('DEBUG: (loop) str: ' + str + ', len: ' + len, ', ch: ' + ch);
    if (len) {
      ch += ch;
    } else {
    // `len` is 0, exit the loop
      break;
    }
  }
  // pad `str`!
  return pad + str;
}