解决了IE8不支持数组的indexOf方法

ie在过去给我们添了很多坑。

 if (!Array.prototype.indexOf) {
     Array.prototype.indexOf = function(elt /*, from*/ ) {
         var len = this.length >>> 0;
         var from = Number(arguments[1]) || 0;
         from = (from < 0) ? Math.ceil(from) : Math.floor(from);
         if (from < 0)
             from += len;
         for (; from < len; from++) {
             if (from in this &&
                 this[from] === elt)
                 return from;
         }
         return -1;
     };
 }