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.

Practice

1. What does an HTTP trigger function in Firebase do?

easy

A. Automatically updates the database without user input

B. Runs code when a specific web URL is visited

C. Sends emails to users without any request

D. Creates a new Firebase project

Solution

1. Step 1: Understand HTTP trigger functions

   HTTP trigger functions run when someone visits a specific web address (URL).

2. Step 2: Compare options with this behavior

   Only Runs code when a specific web URL is visited describes running code on visiting a URL, matching the definition.

3. Final Answer:

   Runs code when a specific web URL is visited -> Option B

4. Quick Check:

   HTTP trigger = run code on URL visit [OK]

Hint: HTTP trigger = runs code on web URL visit [OK]

Common Mistakes:

• Confusing HTTP triggers with database triggers
• Thinking HTTP triggers run automatically without requests
• Assuming HTTP triggers send emails by default

2. Which of the following is the correct way to define an HTTP trigger function in Firebase using Node.js?

easy

A. exports.myFunc = functions.storage.object().onFinalize((object) => { });

B. exports.myFunc = functions.database.onWrite((change, context) => { });

C. exports.myFunc = functions.https.onRequest((req, res) => { res.send('Hello'); });

D. exports.myFunc = functions.auth.user().onCreate((user) => { });

Solution

1. Step 1: Identify HTTP trigger syntax

   HTTP triggers use functions.https.onRequest with (req, res) parameters.

2. Step 2: Match options to this syntax

   Only exports.myFunc = functions.https.onRequest((req, res) => { res.send('Hello'); }); uses functions.https.onRequest correctly; others are for database, auth, or storage triggers.

3. Final Answer:

   exports.myFunc = functions.https.onRequest((req, res) => { res.send('Hello'); }); -> Option C

4. Quick Check:

   HTTP trigger uses functions.https.onRequest [OK]

Hint: HTTP triggers use functions.https.onRequest syntax [OK]

Common Mistakes:

• Using database or auth triggers instead of HTTP triggers
• Missing (req, res) parameters in function
• Confusing storage triggers with HTTP triggers

3. What will be the output when visiting the URL of this Firebase HTTP function?

exports.greet = functions.https.onRequest((req, res) => { const name = req.query.name || 'Guest'; res.send(`Hello, ${name}!`); });

URL visited: https://us-central1-project.cloudfunctions.net/greet?name=Alice

medium

A. Hello, Alice!

B. Hello, Guest!

C. Hello, undefined!

D. Error: name not found

Solution

1. Step 1: Understand query parameter usage

   The function reads the 'name' from the URL query string; if missing, it uses 'Guest'.

2. Step 2: Check the URL query

   The URL has '?name=Alice', so name = 'Alice'. The response is 'Hello, Alice!'.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Query name=Alice -> Hello, Alice! [OK]

Hint: Query params fill variables; missing uses default [OK]

Common Mistakes:

• Ignoring query parameters and using default always
• Assuming undefined if parameter missing
• Expecting error instead of default value

4. Identify the error in this Firebase HTTP trigger function:

exports.testFunc = functions.https.onRequest((request, response) => { response.send('Test'); response.send('Done'); });

medium

A. Incorrect parameter names for the function

B. Missing return statement after response.send()

C. No error, function works fine

D. Calling response.send() twice causes an error

Solution

1. Step 1: Check response.send() usage

   response.send() ends the HTTP response. Calling it twice causes an error.

2. Step 2: Analyze the code flow

   The function calls response.send('Test') then immediately calls response.send('Done'), which is invalid.

3. Final Answer:

   Calling response.send() twice causes an error -> Option D

4. Quick Check:

   Only one response.send() allowed per request [OK]

Hint: Use response.send() only once per HTTP request [OK]

Common Mistakes:

• Calling response.send() multiple times
• Thinking parameter names must be req, res exactly
• Ignoring that response ends after send

5. You want to create an HTTP trigger function that accepts a JSON body with a field age and responds with {"canVote": true} if age is 18 or more, otherwise {"canVote": false}. Which code snippet correctly implements this?

hard

A. exports.checkVote = functions.https.onRequest((req, res) => { const age = req.body.age; if(age >= 18) { res.json({canVote: true}); } else { res.json({canVote: false}); } });

B. exports.checkVote = functions.https.onRequest((req, res) => { const age = req.query.age; if(age > 18) { res.send({canVote: true}); } else { res.send({canVote: false}); } });

C. exports.checkVote = functions.https.onRequest((req, res) => { const age = req.body.age; if(age > 18) { res.send('{"canVote": true}'); } else { res.send('{"canVote": false}'); } });

D. exports.checkVote = functions.https.onRequest((req, res) => { const age = req.body.age; if(age > 18) { res.send({canVote: true}); } else { res.send({canVote: false}); } });

Solution

1. Step 1: Identify correct data source and response method

   The age is sent in JSON body, so use req.body.age. To send JSON response, use res.json().

2. Step 2: Check conditions and response format

   exports.checkVote = functions.https.onRequest((req, res) => { const age = req.body.age; if(age >= 18) { res.json({canVote: true}); } else { res.json({canVote: false}); } }); correctly checks age >= 18 and sends JSON object with canVote true or false using res.json().

3. Final Answer:

   exports.checkVote = functions.https.onRequest((req, res) => { const age = req.body.age; if(age >= 18) { res.json({canVote: true}); } else { res.json({canVote: false}); } }); -> Option A

4. Quick Check:

   Use req.body and res.json() for JSON input/output [OK]

Hint: Use req.body for JSON input and res.json() to respond [OK]

Common Mistakes:

• Using req.query instead of req.body for JSON data
• Sending JSON as string instead of object
• Using res.send() with object without res.json()