IE8 indexOf polyfill from MDN

ie8-indexof.js

// URL: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf

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;
    };
  }