String.prototype.padStart Polyfill

String.prototype.padStart

(function () {
  String.prototype.padStart = String.prototype.padStart || function padStart (targetLength, content) {
    content = String(content)
    
    if (this.length >= targetLength) {
      return this
    }
    
    var needLength = targetLength - this.length;
    var needStr = content;
    
    do {
      needStr = needStr.substr(0, needLength);
      if (needStr.length === needLength) {
        break
      }
      needStr += content
    } while(true);
  }
})()