Transaction read-then-write pattern in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction in Firebase.

Firebase

const result = await db.runTransaction(async (transaction) => {
  const doc = await transaction.get([1]);
  // ...
});

Drag options to blanks, or click blank then click option'

AdbRef

BtransactionRef

CdocRef

DrefDoc

Attempts:

3 left

💡 Hint

Common Mistakes

Using a collection reference instead of a document reference.

Passing the transaction object itself.

The transaction needs a document reference to read data. docRef is the correct variable representing the document.

2fill in blank

medium

Complete the code to check if the document exists inside the transaction.

Firebase

if (!doc.exists) {
  throw new Error([1]);
}

Drag options to blanks, or click blank then click option'

A"Document does not exist"

BDocument does not exist

CdocRef

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the error message string.

Passing a variable instead of a string.

The error message must be a string, so it needs to be in quotes.

3fill in blank

hard

Fix the error in the transaction update call to correctly update the document.

Firebase

transaction.[1](docRef, { count: doc.data().count + 1 });

Drag options to blanks, or click blank then click option'

Aupdate

Bset

Cwrite

Dmodify

Attempts:

3 left

💡 Hint

Common Mistakes

Using set which overwrites the whole document.

Using non-existent methods like write or modify.

The update method updates specific fields in the document without overwriting the whole document.

4fill in blank

hard

Fill both blanks to correctly read and update a user's score inside a transaction.

Firebase

const doc = await transaction.[1](userRef);
transaction.[2](userRef, { score: (doc.data().score || 0) + 10 });

Drag options to blanks, or click blank then click option'

Aget

Bupdate

Cset

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of update which overwrites the document.

Using write which is not a valid method.

First, you read the document with get. Then, you update the score field with update.

5fill in blank

hard

Fill all three blanks to complete a transaction that increments a product's stock count safely.

Firebase

const productDoc = await transaction.[1](productRef);
if (!productDoc.exists) {
  throw new Error([2]);
}
transaction.[3](productRef, { stock: productDoc.data().stock + 1 });

Drag options to blanks, or click blank then click option'

Aget

B"Product does not exist"

Cupdate

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of update which overwrites the document.

Throwing an error without quotes around the message.

Trying to update before reading the document.

Use get to read the product document, throw an error with a string message if it doesn't exist, then update the stock count.