gcm-courier.js

function GCMCourier(apiKey) {
	this.apiKey = apiKey;
	this.push = function(notification, callback) {
		var xhttp = new XMLHttpRequest();
		xhttp.onreadystatechange = function() {
			if (xhttp.readyState==4) {
				if (xhttp.status == 200) {
					var output = xhttp.responseText;
					callback && callback(true, output);
				} else {
					callback && callback(false, xhttp.responseText, xhttp.status);
				}
			}
		}
		xhttp.open('POST', 'https://gcm-http.googleapis.com/gcm/send', true);
		xhttp.setRequestHeader('Content-Type', 'application/json');
		xhttp.setRequestHeader('Authorization', 'key='+apiKey);
		xhttp.send(JSON.stringify(notification));
	};
};

Usage:

var courier = new GCMCourier(<GCM secret's key>);
var payload = {
    to: '<device_token>',
    content_available: true,   // for iOS
    notification: {
        title: 'hello',
        body: 'this is from javascript!'
    }
};
courier.push(payload, console.log.bind(console));

For more info please refer to: http://peatiscoding.me/geek-stuff/javascript-to-deliver-gcm-payload/