I'm getting the error 'foo' is undefined. in my script when i test my function with an undefined parameter. As far as I understand, This shouldn't be happening.

My calling code:

//var foo
var test = peachUI().stringIsNullOrEmpty(foo) ;

My function (part of a larger framework).

    stringIsNullOrEmpty: function (testString) {
        /// <summary>
        ///     Checks to see if a given string is null or empty.
        /// </summary>
        /// <param name="testString" type="String">
        ///     The string check against.
        /// </param>
        /// <returns type="Boolean" />

        var $empty = true;

        if (typeof testString !== "undefined") {
            if (testString && typeof testString === "string") {

                if (testString.length > 0) {
                    $empty = false;
                }

            }
        }
        return $empty;
    }

Any ideas?

Please note. I've had a good read of other similar questions before posting this one.