denoland-deno/std/node/tests/node_modules/left-pad/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;
  // 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))
    if (len) ch += ch;
    // `len` is 0, exit the loop
    else break;
  }
  // pad `str`!
  return pad + str;
}