title: Sending push notification to a specific user in cloud function FCM
body: <p><strong>Please note that this question is not about getting device token on client side.</strong></p>
  <p>I am very new to <code>react-native</code> and <code>firebase</code> and I want to implement push notication service of FCM. What I am planning to do is to send a notification to particular device using its device token <strong>on my nodejs app server or cloud function( not through console)</strong>. But as I have found, FCM doesn't provides any API to accrss token by username. Suppose I want to send notification to user X( for the time being, suppose that one user signs only on single device). Now using the function <code>sendToDevice()</code>, I can send the message to a specific device. But how would I know the device token of user X. Do I need to store tokens in the firebase database by myself? Or can I get along without storing FCM tokens?</p>
  <p>Please guide me because firebase docs aren't clear about this.</p>