I'm trying to use the Haversine Distance Formula (as found here: http://www.movable-type.co.uk/scripts/latlong.html) but I can't get it to work, please see the following code

    function test() { 

    var lat2 = 42.741; 

    var lon2 = -71.3161; 

    var lat1 = 42.806911; 

    var lon1 = -71.290611; 



    var R = 6371; // km 

    //has a problem with the .toRad() method below.

    var dLat = (lat2-lat1).toRad();  

    var dLon = (lon2-lon1).toRad();  

    var a = Math.sin(dLat/2) * Math.sin(dLat/2) + 

                    Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) * 

                    Math.sin(dLon/2) * Math.sin(dLon/2);  

    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 

    var d = R * c; 



    alert(d); 

}

And the error is:

Uncaught TypeError: Object -0.06591099999999983 has no method 'toRad' 

Which I understand to be because it needs to do the following:

Number.prototype.toRad = function() {

return this * Math.PI / 180;

}

But when I put this below the function, it still comes back with the same error message. How do I make it use the helper method? Or is there an alternative way to code this to get it to work? Thanks!