Calculate heading from progression of latitude and longitude.

azimuth.js

class Azimuth {
	constructor() {
		this.watchId = null;
		this.latitude = null;
		this.longitude = null;

		this.latLon = (lat, lon) => {
			this.latitude = lat;
			this.longitude = lon;
		};
		this.loc = {
			origin: new this.latLon(null, null),
			destination: new this.latLon(null, null)
		};
		this.calculateBearing = () => {
			// origination point latitude
			const ϕ1 = this.loc.origin.latitude.toRadians();
			// origin point longitude
			const λ1 = this.loc.origin.longitude;
			// destination point latitude
			const ϕ2 = this.loc.destination.latitude.toRadians();
			// destination point longitude
			const λ2 = this.loc.destination.longitude;
			// diff between longitudes (longitudinal delta)
			const Δλ = (λ2 - λ1).toRadians();
			// y-axis angle in radians
			const y = Math.sin(Δλ) * Math.cos(ϕ2);
			// x-axis angle in radians
			const x = Math.cos(ϕ1) * Math.sin(ϕ2) - Math.sin(ϕ1) * Math.cos(ϕ2) * Math.cos(Δλ);
			// bearing in degrees (arc angle)
			const θ = Math.atan2(y, x).toDegrees();
			// bearing - adjusted to translate positive and negative bearing/heading to the degrees of a compass/circle.
			const β = (θ + 360) % 360;

			return β;
		};
		this.setCoords = data => {
			const { coords } = data;

			// set previous lat/lon as current (stored) lat/lon
			this.loc.origin.latitude = this.loc.destination.latitude;
			this.loc.origin.longitude = this.loc.destination.longitude;

			// set current lat/lon with current system-provided location data
			this.loc.destination.latitude = coords.latitude;
			this.loc.destination.longitude = coords.longitude;

			!!this.loc.origin.latitude && this.calculateBearing();
		};

		this.doLocation = () => {
			// location is a reserved keyword
			navigator.geolocation.getCurrentPosition(lokation => {
				if (!lokation.corrds) {
					this.stopWatchingLocation();
				}
				this.setCoords(lokation);
			}, this.stopWatchingLocation);
		};

		this.startWatchingLocation = () => {
			this.watchID = setInterval(doLocation, 2000);
		};

		this.stopWatchingLocation = err => {
			clearInterval(this.watchID);
			if (err && err.error && err.error.code && err.error.message) {
				console.error(err.error.code + ' ' + err.error.message);
			}
			return false;
		};
	}
}