//判断当前字符串是否以str开始 先判断是否存在function是避免和js原生方法冲突，自定义方法的效率不如原生的高
     if (typeof String.prototype.startsWith != 'function'){
       String.prototype.startsWith = function (str){
          return this.slice(0, str.length) == str;
       };
     }
　　　　
     //判断当前字符串是否以str结束
     if (typeof String.prototype.endsWith != 'function') {
       String.prototype.endsWith = function (str){
          return this.slice(-str.length) == str;
       };
     }

     //测试程序
     var sCompareStr = "abcdefg123";//比较字符串
     var sBeCompareStr = "fg";//被比较字符串
     if(sCompareStr.endsWith(sBeCompareStr)){//这里可以替换为startsWith
          alert(sCompareStr+" 是以:"+sBeCompareStr+" 结束");
     }else{
          alert(sCompareStr+" 不是以:"+sBeCompareStr+" 结束");
     }