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

Process Flow - HTTP trigger functions

HTTP Request Received

↓

Firebase Function Triggered

↓

Function Executes Code

↓

Prepare HTTP Response

↓

Send Response to Client

↓

Function Ends

When an HTTP request arrives, Firebase runs the function code, then sends back a response.

Execution Sample

Firebase

const functions = require('firebase-functions');

exports.helloWorld = functions.https.onRequest((req, res) => {
  res.send('Hello from Firebase!');
});

This function replies with a greeting when triggered by an HTTP request.

Process Table

Step	Action	Input	Output	Notes
1	Receive HTTP request	Client sends GET /hello	Request object created	Function triggered by request
2	Execute function code	Request object	Prepare response	Function runs callback
3	Send response	Response object	'Hello from Firebase!'	Client receives greeting
4	Function ends	null	Cleanup	Function completes execution

💡 Function ends after sending HTTP response to client

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
req	undefined	Request object	Request object	Request object	undefined after function ends
res	undefined	Response object	Response object	Response sent	undefined after function ends

Key Moments - 2 Insights

Why does the function end after sending the response?

Because HTTP functions must send a response to complete; see execution_table step 3 where response is sent, then step 4 ends function.

What triggers the function to run?

An HTTP request triggers it, as shown in execution_table step 1 when the request arrives.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 3?

A Cleanup

B 'Hello from Firebase!'

C Request object

D Response object

💡 Hint

Check the Output column at step 3 in the execution_table

At which step does the function receive the HTTP request?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the Action and Input columns in execution_table step 1

If the function did not send a response, what would happen?

A Client would wait indefinitely

B Function would end normally

C Response would be sent automatically

D Function would restart

💡 Hint

Refer to key_moments about why sending a response is necessary

Concept Snapshot

HTTP trigger functions run when an HTTP request arrives.
They receive request and response objects.
Function code runs and sends a response.
Sending a response ends the function.
Used to build APIs or webhooks.

Full Transcript

When an HTTP request comes to Firebase, it triggers the HTTP function. The function receives a request object representing the incoming data and a response object to send data back. The function runs its code, usually preparing a message or data to send. Then it sends the response back to the client. After sending the response, the function ends. If no response is sent, the client waits and the function may time out. This flow allows Firebase to handle web requests easily.