Progressive internet 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.information.information); // 4
  registration.sync.register('task-sync'); // 5
};

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

persistTask = operate(process){ // 14
  let transaction = db.transaction("duties", "readwrite");
  let tasksObjectStore = transaction.objectStore("duties");
  let addRequest = tasksObjectStore.add(process);
  addRequest.onsuccess = operate(occasion){ console.log("Activity added to DB"); };
  addRequest.onerror = operate(occasion) { console.log(“Error: “ + occasion); };
}
self.addEventListener('sync', async operate(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 = operate(occasion) { // 17
        let cursor = occasion.goal.consequence;
        if (cursor) {
          let process = cursor.worth; // 18
          fetch(URL + 'todos/add', // a
            { technique: 'POST', 
              headers: { 'Content material-Kind': 'software/json' },
              physique: JSON.stringify({ "process" : process }) 
            }).then((serverResponse) => {
              console.log("Activity saved to backend.");
              deleteTasks(); // b
              res(); // b
            }).catch((err) => {
              console.log("ERROR: " + err);
              rej(); //c
            })
          }
        } 
    }))
  }
})
async operate 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 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 will pay attention for messages.
3. Right here, we’re anticipating task-channel message occasions. In responding to those occasions, we do two issues:
4. Name persistTask() to avoid wasting the brand new process 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’s going to retry when the community is accessible, and implements a again off technique and give-up situations.
6. With that carried out, we create a reference for our database object.
7. Receive a “request” for the deal with on our database. All the pieces on IndexedDB is dealt with asynchronously. (For a wonderful overview of IndexedDB, I like to recommend this sequence.)
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 international db object.
10. If the duties assortment isn’t current, we create the duties assortment.
11. If the database was efficiently created, we reserve it to our db object.
12. Log the error if the database creation failed.
13. The persistTask() operate referred to as by the add-task broadcast occasion (4). This merely places the brand new process worth within the duties assortment.
14. Our sync occasion. That is referred to as by the published occasion (5). We verify for the occasion.tag subject being task-sync so we all know it’s our task-syncing occasion.
15. occasion.waitUntil() permits us to inform the serviceWorker that we aren’t carried out 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 preserve making an attempt. Additionally, keep in mind that if the community is unavailable, it can wait till it turns into accessible.
    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 receive 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 have now a variable with the worth of our broadcast process in it. With that in hand:
    1. We difficulty 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 may reject the containing promise, letting the Sync API know the request should 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 each time our community is spotty. Bear in mind, we’re getting this within the browser, throughout all types of gadgets, cellular and in any other case.

Testing the PWA instance

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