Firestore trigger functions in Firebase - Step-by-Step Execution

Process Flow - Firestore trigger functions

Event in Firestore

↓

Trigger Function Activated

↓

Read Event Data

↓

Perform Logic (e.g., update, notify)

↓

Write Back or Side Effects

↓

Function Ends

When a Firestore event happens, the trigger function runs, reads event data, does work, then finishes.

Execution Sample

Firebase

exports.onUserCreate = functions.firestore
  .document('users/{userId}')
  .onCreate((snap, context) => {
    const newUser = snap.data();
    console.log('New user:', newUser.name);
  });

This function runs when a new user document is created and logs the user's name.

Process Table

Step	Event	Trigger Function Action	Data Accessed	Output/Effect
1	New document created in 'users/abc123'	Function onUserCreate activates	snap.data() returns {name: 'Alice'}	Logs: 'New user: Alice'
2	Function completes	No further action	No new data	Function ends

💡 Function ends after processing the create event and logging user name

Status Tracker

Variable	Start	After Step 1	Final
snap	undefined	DocumentSnapshot	DocumentSnapshot
newUser	undefined	{name: 'Alice'}	{name: 'Alice'}

Key Moments - 2 Insights

Why does the function only run when a new document is created?

Because the trigger uses .onCreate(), it activates only on document creation events as shown in execution_table step 1.

What is snap.data() and why do we use it?

snap.data() gives the content of the changed document. In step 1, it returns the new user's data so we can use it inside the function.

Visual Quiz - 3 Questions

Look at the execution table, what data does snap.data() return at step 1?

A undefined

B {name: 'Alice'}

C {userId: 'abc123'}

D null

💡 Hint

Check the 'Data Accessed' column in execution_table row for step 1

At which step does the function finish running?

A Step 1

B Before Step 1

C Step 2

D Never finishes

💡 Hint

Look at the 'Output/Effect' column in execution_table for step 2

If the trigger was changed to .onUpdate(), when would the function run?

A When a document is updated

B When a document is deleted

C When a document is created

D When Firestore starts

💡 Hint

Recall the meaning of .onUpdate() triggers in Firestore functions

Concept Snapshot

Firestore trigger functions run automatically on database events.
Use .onCreate(), .onUpdate(), or .onDelete() to specify event type.
Function receives snapshot with event data.
Perform logic inside function, then finish.
Useful for reacting to data changes in real time.

Full Transcript

Firestore trigger functions activate automatically when specific events happen in your Firestore database, like creating, updating, or deleting documents. For example, a function with .onCreate() runs only when a new document is added. Inside the function, you get a snapshot of the changed document's data using snap.data(). You can then perform actions like logging, updating other data, or sending notifications. After the function finishes its work, it ends. This lets your app respond instantly to database changes without manual checks.