accessing iPhone compass with JavaScript

You cannot access that information via javascript, unless you're using something like iPhoneGap

At the time this was true, in iOS 5 you can use the compass heading in JS. https://developer.apple.com/documentation/webkitjs/deviceorientationevent/1804777-webkitcompassheading

On iOS, you can retrieve the compass value like this.

window.addEventListener('deviceorientation', function(e) {
    console.log( e.webkitCompassHeading );
}, false);

For more informations, read the Apple DeviceOrientationEvent documentation.

Hope this helps.