CODE EXAMPLE FOR JAVASCRIPT

fcm node

var FCM = require('fcm-node');
var serverKey = 'YOURSERVERKEYHERE'; //put your server key here
var fcm = new FCM(serverKey);

fcm.subscribeToTopic([ 'device_token_1', 'device_token_2' ], 'some_topic_name', (err, res) => {
    assert.ifError(err);
    assert.ok(res);
    done();
});