As a primer you should review this previous related StackOverflow answer and be familiar with the general practical considerations for using DeviceOrientation Events in web applications.
The simple solution I provided in my previous related StackOverflow answer only applies to browsers that implement absolute deviceorientation events (i.e. browsers where the deviceorientation alpha property is compass-oriented). That means the solution provided there currently only works in Android browsers and not iOS-based browsers or any other browser that does not provide absolute-based deviceorientation event data.
To reliably obtain the current compass heading across both Android and iOS browsers today you need to handle both absolute and non-absolute implementations that provide the additional webkitCompassHeading property and make sure to account for any current screen orientation changes as part of that. AFAIK the only library that currently does this is Full Tilt JS (disclaimer: I am the author of this library).
The following code will give you the same correct compass heading across both iOS and Android browsers, taking account of the differences in device orientation implementations and applying any necessary runtime screen orientation transforms too:
<!-- Include the Full Tilt JS library from https://github.com/richtr/Full-Tilt-JS -->
<script src="https://stackoverflow.com/questions/16048514/fulltilt-min.js"></script>
<script>
// Obtain a new *world-oriented* Full Tilt JS DeviceOrientation Promise
var promise = FULLTILT.getDeviceOrientation({ 'type': 'world' });
// Wait for Promise result
promise.then(function(deviceOrientation) { // Device Orientation Events are supported
// Register a callback to run every time a new
// deviceorientation event is fired by the browser.
deviceOrientation.listen(function() {
// Get the current *screen-adjusted* device orientation angles
var currentOrientation = deviceOrientation.getScreenAdjustedEuler();
// Calculate the current compass heading that the user is 'looking at' (in degrees)
var compassHeading = 360 - currentOrientation.alpha;
// Do something with `compassHeading` here...
});
}).catch(function(errorMessage) { // Device Orientation Events are not supported
console.log(errorMessage);
// Implement some fallback controls here...
});
</script>
Here is a demo that demonstrates this technique to obtain the compass heading the user is facing. It should work well on both iOS and Android browsers.
The implementation of the code in that demo is as shown above and can be viewed on Github at ./scripts/compass.js:L268-L272.