Push Notification notifyUser

notifyUser.ts

/**
 * Function to send a Push Notification using Firebase Messaging.
 * @param userID The user to receive the push message.
 * @param taskCount The amount of open tasks.
 * @returns Returns the message ID or undefined in case of failure.
 */
function notifyUser(userID: string, taskCount: number): Promise<string | undefined> {
    // Again we're just initiating a new Promise. 
    // A closure that can succeed and fail.
    return new Promise(async (resolve, reject) => {
        // This function just gets us the user based on the userID.
        // There's no value in me providing this function.
        // It's just getting the data associated with an ID from the database.
        const user = await getUser(userID)
        
        // Next we need to check, if the user exists
        // AND -> If they registered for push notifications, or not.
        // The `gcmToken` is what we sent from the mobile application to the server
        if (!user || !user.gcmToken) {
            // If we end up here, then we want to pass undefined 
            // (aka nil) to the closure that expects a message ID
            resolve(undefined)
            return
        }

        // The following is the actual payload.
        // Most of it is boilerplate code. 
        // Check out firebase.messaging.Message for more.
        // https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages
        //
        // `titleLocKey`, `locKey` and `locArgs` are the only relevant fields for us.
        // `token` defines who the receiver of the Notification will be. 
        const payload = {
            apns: {
                payload: {
                    aps: {
                        alert: {
                            titleLocKey: "OPEN_TASKS_HEADER",
                            locKey: "OPEN_TASKS_BODY",
                            locArgs: [
                                `${taskCount}`
                            ]
                        },
                        "mutable-content": false,
                        "thread-id": "open-tasks",
                        badge: taskCount
                    },
                }
            },
            token: user.gcmToken
        }

        // The rest is just boilerplate.
        // Using Firebase's messaging object, to send the payload.
        messaging.send(payload)
            .then((response: string) => {   
                // Handle this however you see fit.
                // The message was sent successfully.
                resolve(`Did send message to: ${userID} about ${taskCount} task/s ID: ${response}`)
            })
            .catch((error: Error) => {
                // The message was not sent.
                // You need to check the error and debug why.
                reject(error)
            });
    })
}