leftpad-test.js

'use strict';

function leftpad1(str, len, ch) {
  str = String(str);

  var i = -1;

  if (!ch && ch !== 0) ch = ' ';

  len = len - str.length;

  while (++i < len) {
    str = ch + str;
  }

  return str;
}

function leftpad2(str, len, ch) {
  str = String(str);

  if (!ch && ch !== 0) ch = ' ';

  return String(ch).repeat(len).substring(str.length) + str;
}

function leftpad3(str, len, ch) {
  str = String(str);

  var i = -1;

  if (!ch && ch !== 0) ch = ' ';

  len = len - str.length;
  if (len > 0) {
    var arr = new Array(len);
    arr.fill(ch);
    return arr.join('') + str;
  } else {
    return str;
  }
}

const len = parseInt(process.argv[2], 10);
const count = 1000000;

console.log("pad length", len);

console.time('concat with +');
for (let i = 0; i < count; i++) {
    leftpad1('aaaa', len, 0);
}
console.timeEnd('concat with +');

console.time('repeat substring');
for (let i = 0; i < count; i++) {
    leftpad2('aaaa', len, 0);
}
console.timeEnd('repeat substring');

console.time('array fill and join');
for (let i = 0; i < count; i++) {
    leftpad3('aaaa', len, 0);
}
console.timeEnd('array fill and join');