In IE8 arrays don't have the method indexOf defined. Implementation taken from MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf Other source could be: http://ecma262-5.com/ELS5_HTML.htm#Section_15.4.4.14

array_indeof.js

if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function (searchElement, fromIndex) {
    if ( this === undefined || this === null ) {
      throw new TypeError( '"this" is null or not defined' );
    }
    var length = this.length >>> 0; // Hack to convert object.length to a UInt32
    fromIndex = +fromIndex || 0;
    if (Math.abs(fromIndex) === Infinity) {
      fromIndex = 0;
    }
    if (fromIndex < 0) {
      fromIndex += length;
      if (fromIndex < 0) {
        fromIndex = 0;
      }
    }
    for (;fromIndex < length; fromIndex++) {
      if (this[fromIndex] === searchElement) {
        return fromIndex;
      }
    }
    return -1;
  };
}