How to fix Array indexOf() in JavaScript for Internet Explorer browsers

Do it like this...

if (!Array.prototype.indexOf) {

}

As recommended compatibility by MDC.

In general, browser detection code is a big no-no.

Alternatively, you could use the jQuery 1.2 inArray function, which should work across browsers:

jQuery.inArray( value, array [, fromIndex ] )

The full code then would be this:

if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function(obj, start) {
         for (var i = (start || 0), j = this.length; i < j; i++) {
             if (this[i] === obj) { return i; }
         }
         return -1;
    }
}

For a really thorough answer and code to this as well as other array functions check out Stack Overflow question Fixing JavaScript Array functions in Internet Explorer (indexOf, forEach, etc.).