string 扩展方法

在操作字符串（String）类型的时候，startsWith(anotherString)和endsWith(anotherString)是非常好用的方法。其中startsWith判断当前字符串是否以anotherString作为开头，而endsWith则是判断是否作为结尾。举例：

 if (!String.prototype.endsWith)
   String.prototype.endsWith = function(searchStr, Position) {
       // This works much better than >= because
       // it compensates for NaN:
       if (!(Position < this.length))
         Position = this.length;
       else
         Position |= 0; // round position
       return this.substr(Position - searchStr.length,
                          searchStr.length) === searchStr;
   };


   if (typeof String.prototype.endsWith != 'function') {
    String.prototype.endsWith = function(suffix) {
     return this.indexOf(suffix, this.length - suffix.length) !== -1;
    };
   }

 if (typeof String.prototype.startsWith != 'function') {
  String.prototype.startsWith = function (prefix){
   return this.slice(0, prefix.length) === prefix;
  };
 }

计算字符串长度(英文占1个字符，中文汉字占2个字符)

String.prototype.gblen = function() {
  var len = 0;
  for (var i=0; i127 || this.charCodeAt(i)==94) {
       len += 2;
     } else {
       len ++;
     }
   }
  return len;
}