判断数组是否支持indexOf

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 ;

    }

    return -1;

  };

}