Detect installOrigin in Firefox apps to defer preload status.

detectInstallOrigin.js

/**
 * Detect installOrigin in Firefox apps to defer preload status.
 * 
 * For most apps the installOrigin will be https://marketplace.firefox.com
 * Preloaded apps *can* customize their installOrigin to reflect country and/or
 * carrier (like https://fxos.telefonia.es for Telefónica in Spain).
 *
 * @param  {Function} done Node-style callback (err, origin)
 * @example
 *   detectInstallOrigin(function(err, origin) {
 *     if (!origin) {
 *       // Nothing to track
 *       return;
 *     }
 *     // We could override the referrer to the origin in Google analytics
 *       _gaq.push(['_setReferrerOverride', origin]);
 *     }
 *     
 *     // Or set a custom var on the visitor level
 *     _gaq.push(['_setCustomVar', 1, 'InstallOrigin', origin, 1]);
 *     
 *     // Or redirect to another site
 *     var href = '/carrier-code?utm_source=' + encodeURIComponent(origin);
 *     location.href = href;
 *   });
 * @author Harald Kirschner <harald@mozilla.com>
 */
function detectInstallOrigin(done) {
	if (!navigator.mozApps) {
		done('mozApps not supported');
	}
	var request = navigator.mozApps.getSelf();
	request.onsuccess = function() {
		if (request.result) {
			done(null, request.result.installOrigin);
		} else {
			done('App not installed');
		}
	};
	request.onerror = function() {
		done('Error: ' + request.error.name);
	};
}