像这样简单的事情可以做到这一点：

var request = require('request');
 
function sendMessageToUser(deviceId,message) {
  request({
    url: 'https://fcm.googleapis.com/fcm/send',method: 'POST',headers: {
      'Content-Type' :' application/json','Authorization': 'key=AI...8o'
    },body: JSON.stringify(
      { "data": {
        "message": message
      },"to" : deviceId
      }
    )
  },function(error,response,body) {
    if (error) { 
      console.error(error,body); 
    }
    else if (response.statusCode >= 400) { 
      console.error('HTTP Error: '+response.statusCode+' - '+response.statusMessage+'\n'+body); 
    }
    else {
      console.log('Done!')
    }
  });
 
sendMessageToUser(
  "d7x...KJQ",{ message: 'Hello puf'}
);

更新(2017年4月)：现在,您还可以在Cloud Functions for Firebase中运行与此相似的代码.见https://firebase.google.com/docs/functions/use-cases#notify_users_when_something_interesting_happens