Progressive net app necessities: Service employee background sync

const URL = "https://8014-35-223-70-178.ngrok-free.app/"; // 1
const taskChannel = new BroadcastChannel('task-channel'); // 2
taskChannel.onmessage = occasion => { // 3
  persistTask(occasion.knowledge.knowledge); // 4
  registration.sync.register('task-sync'); // 5
};

let db = null; // 6
let request = indexedDB.open("TaskDB", 1); // 7
request.onupgradeneeded = perform(occasion) { // 8
  db = occasion.goal.end result; // 9
  if (!db.objectStoreNames.incorporates("duties")) { // 10
    let tasksObjectStore = db.createObjectStore("duties", { autoIncrement: true }); // 11
  }
};
request.onsuccess = perform(occasion) { db = occasion.goal.end result; }; // 12
request.onerror = perform(occasion) { console.log("Error in db: " + occasion); }; // 13

persistTask = perform(activity){ // 14
  let transaction = db.transaction("duties", "readwrite");
  let tasksObjectStore = transaction.objectStore("duties");
  let addRequest = tasksObjectStore.add(activity);
  addRequest.onsuccess = perform(occasion){ console.log("Process added to DB"); };
  addRequest.onerror = perform(occasion) { console.log(“Error: “ + occasion); };
}
self.addEventListener('sync', async perform(occasion) { // 15
  if (occasion.tag == 'task-sync') {
    occasion.waitUntil(new Promise((res, rej) => { // 16
      let transaction = db.transaction("duties", "readwrite");
      let tasksObjectStore = transaction.objectStore("duties");
      let cursorRequest = tasksObjectStore.openCursor();
      cursorRequest.onsuccess = perform(occasion) { // 17
        let cursor = occasion.goal.end result;
        if (cursor) {
          let activity = cursor.worth; // 18
          fetch(URL + 'todos/add', // a
            { technique: 'POST', 
              headers: { 'Content material-Kind': 'software/json' },
              physique: JSON.stringify({ "activity" : activity }) 
            }).then((serverResponse) => {
              console.log("Process saved to backend.");
              deleteTasks(); // b
              res(); // b
            }).catch((err) => {
              console.log("ERROR: " + err);
              rej(); //c
            })
          }
        } 
    }))
  }
})
async perform deleteTasks() { // 19
  const transaction = db.transaction("duties", "readwrite");
  const tasksObjectStore = transaction.objectStore("duties");
  tasksObjectStore.clear();
  await transaction.full;
}

Now let’s speak about what is going on on this code.

1. We have to route our requests by way of the identical safe tunnel we created with ngrok, so we save the URL right here.
2. Create the published channel with the identical identify so we are able to pay attention for messages.
3. Right here, we’re waiting for task-channel message occasions. In responding to those occasions, we do two issues:
4. Name persistTask() to avoid wasting the brand new activity to IndexedDB.
5. Register a brand new sync occasion. That is what invokes the particular functionality for retrying requests intelligently. The sync handler permits us to specify a promise that it’ll retry when the community is accessible, and implements a again off technique and give-up circumstances.
6. With that accomplished, we create a reference for our database object.
7. Acquire a “request” for the deal with on our database. Every part on IndexedDB is dealt with asynchronously. (For a superb overview of IndexedDB, I like to recommend this collection.)
8. The onupgradeneeded occasion fires if we’re accessing a brand new or up-versioned database. 
9. Inside onupgradeneeded, we get a deal with on the database itself, with our world db object.
10. If the duties assortment shouldn’t be current, we create the duties assortment.
11. If the database was efficiently created, we put it aside to our db object.
12. Log the error if the database creation failed.
13. The persistTask() perform known as by the add-task broadcast occasion (4). This merely places the brand new activity worth within the duties assortment.
14. Our sync occasion. That is known as by the published occasion (5). We examine for the occasion.tag discipline being task-sync so we all know it’s our task-syncing occasion.
15. occasion.waitUntil() permits us to inform the serviceWorker that we’re not accomplished till the Promise inside it completes. As a result of we’re in a sync occasion, this has particular that means. Particularly, if our Promise fails, the syncing algorithm will hold making an attempt. Additionally, keep in mind that if the community is unavailable, it can wait till it turns into obtainable.
    1. We outline a brand new Promise, and inside it we start by opening a connection to the database.
16. Inside the database onsuccess callback, we get hold of a cursor and use it to seize the duty we saved. (We’re leveraging our wrapping Promise to cope with nested asynchronous calls.)
17. Now we’ve a variable with the worth of our broadcast activity in it. With that in hand:
    1. We problem a brand new fetch request to our expressJS /todos/add endpoint.
    2. Discover that if the request succeeds, we delete the duty from the database and name res() to resolve our outer promise.
    3. If the request fails, we name rej(). This can reject the containing promise, letting the Sync API know the request have to be retried.
18. The deleteTasks() helper technique deletes all of the duties within the database. (This can be a simplified instance that assumes one duties creation at a time.)

Clearly, there’s a lot to this, however the reward is having the ability to effortlessly retry requests within the background at any time when our community is spotty. Bear in mind, we’re getting this within the browser, throughout every kind of gadgets, cell and in any other case.

Testing the PWA instance

In case you run the PWA now and create a to-do, it’ll be despatched to the again finish and saved. The attention-grabbing take a look at is to open devtools (F12) and disable the community. You could find the “Offline” choice within the “throttling” menu of the community tab like so: