Alright, so in Firebase Firestore, a batched write is basically a group of write operations on one or more documents. The cool thing about batched writes is their atomicity. This means that the whole batch either goes through or none of it does. If one part of the batch fails, the entire thing fails, and the database stays unchanged. Pretty neat, right?

• Step 1: First things first, you need to create a write batch. In JavaScript, it looks like this:

var db = firebase.firestore();
var batch = db.batch();

• Step 2: Now, let's add some write operations to the batch. You need to know which documents you're going to write to. If you're modifying an existing document, you need to create a DocumentReference first. Here's an example where we're setting and updating some documents:

var docRef1 = db.collection("collection_1").doc("document_1");
var docRef2 = db.collection("collection_2").doc("document_2");
var docRef3 = db.collection("collection_3").doc("document_3");

// Set the value of 'document_1'
batch.set(docRef1, {foo: "bar"});
   
// Update the value of 'document_2'
batch.update(docRef2, {"name": "new_name"});          
  
// Delete 'document_3'
batch.delete(docRef3);

• Step 3: Finally, commit the batch with batch.commit(). Remember, nothing is written to the database until you do this step:

batch.commit().then(function() {
  console.log("Batch write completed successfully.");
}).catch(err => {
  console.log("Batch write failed: ", err);
});

The .commit() method returns a Promise that resolves when the write finishes. You can use this to confirm that everything went smoothly or to handle any errors that pop up.

As we talked about earlier, Firestore's batch writes guarantee atomicity. This means if any part of the batch fails, none of the operations in the batch are applied. This helps keep your data correct and consistent. If something goes wrong, Firestore throws an error, and you can handle it in your catch block. This automatic rollback prevents partial updates and inconsistent states, which is super important for keeping your data safe and sound.