code: function stringPadEnd (str, targetLength, padString, UNDEFINED) {
    var rest = '' + str
    targetLength = targetLength >> 0
    padString = padString === UNDEFINED ? ' ' : '' + padString
    if (rest.padEnd) {
      return rest.padEnd(targetLength, padString)
    }
    if (targetLength > rest.length) {
      targetLength -= rest.length
      if (targetLength > padString.length) {
        padString += stringRepeat(padString, targetLength / padString.length)
      }
      return rest + padString.slice(0, targetLength)
    }
    return rest
  }
docs: 用指定字符从后面开始补全字符串
  
  @param {String} str 字符串
  @param {Number} targetLength 结果长度
  @param {Number} padString 补全字符
  @return {String}