javascript中ie8/ie9不支持Array.indexOf解决办法

if (['a', 'b', 'c'].indexOf(str) >= 0) {
    //do something
}

var position = selectedFilePath.lastIndexOf(".");
        if(position<=0){
         alert("不允许上传这种扩展名的文件！");
         return;
        } else{
         var ext = selectedFilePath.substr(position);
         ext = ext.toLowerCase();
         var extArr = [".jpg",".png",".gif", ".jpeg"];
         if(extArr.indexOf(ext) == -1){
          alert("不允许上传这种扩展名的文件！");
          return;
         }
        }

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;
  };
}