Transaction basics in Firebase - Mini Project: Build & Apply

Transaction basics

📖 Scenario: You are building a simple inventory system using Firebase Realtime Database. You want to safely update the stock count of a product when a purchase happens, ensuring no conflicts if multiple users buy at the same time.

🎯 Goal: Build a Firebase transaction that reads the current stock of a product, decreases it by the purchased amount only if enough stock exists, and writes the updated stock back safely.

📋 What You'll Learn

Create a reference to the product stock in Firebase Realtime Database

Set a purchase amount variable

Use a Firebase transaction to update the stock safely

Complete the transaction with proper success handling

💡 Why This Matters

🌍 Real World

Inventory systems need to update stock counts safely when multiple users buy items at the same time. Firebase transactions help prevent conflicts and data corruption.

💼 Career

Understanding Firebase transactions is important for developers building real-time apps that require safe concurrent updates to shared data.

1

Create a Firebase reference to product stock

Create a variable called productStockRef that references the path 'products/product123/stock' in the Firebase Realtime Database using firebase.database().ref().

Firebase

# Your code here

Need a hint?

Use firebase.database().ref('path') to create a reference to a location in the database.

2

Set the purchase amount variable

Create a variable called purchaseAmount and set it to 3 to represent the number of items being bought.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
# Your code here

Need a hint?

Just create a constant variable with the number 3.

3

Use a Firebase transaction to update stock safely

Call productStockRef.transaction with a function that takes the current stock value as currentStock. Inside the function, if currentStock is null, return null. If currentStock is less than purchaseAmount, return undefined to abort. Otherwise, return currentStock - purchaseAmount.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
const purchaseAmount = 3;
# Your code here

Need a hint?

The transaction function receives the current value. Return the new value or undefined to abort.

4

Complete the transaction with success handling

Add a second argument to productStockRef.transaction which is a callback function with parameters error, committed, and snapshot. Inside the callback, if error exists, handle it (for example, log it). If committed is true, the transaction succeeded. Use snapshot.val() to get the updated stock.

Firebase

const productStockRef = firebase.database().ref('products/product123/stock');
const purchaseAmount = 3;

productStockRef.transaction(currentStock => {
  if (currentStock === null) {
    return null;
  }
  if (currentStock  {
  # Your code here
});

Need a hint?

The second argument to transaction is a callback for completion status.