HTTP trigger functions in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Firebase HTTP trigger function?

Consider this Firebase HTTP trigger function code snippet:

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

What will the client receive when making a GET request to this function?

Firebase

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

A The client receives an error because the function does not return a promise.

B The client receives a JSON object {message: 'Hello from Firebase!'} as the response body.

C The client receives an empty response with status 200.

D The client receives the string 'Hello from Firebase!' as the response body.

💡 Hint

Look at what the response.send() method sends back.

The response.send() method sends the exact string as the HTTP response body. Here, it sends 'Hello from Firebase!'.

📝 Syntax

intermediate

Identify the syntax error in this Firebase HTTP trigger function

Which option contains the correct syntax for defining a Firebase HTTP trigger function?

Firebase

exports.myFunction = functions.https.onRequest((req, res) => {
  res.send('Hi');
});

A

exports.myFunction = functions.https.onRequest((req, res) => {
  res.send('Hi')
});

B

exports.myFunction = functions.https.onRequest((req, res) => {
  res.send('Hi');
});

C

exports.myFunction = functions.https.onRequest(req, res) => {
  res.send('Hi');
};

D

exports.myFunction = functions.https.onRequest = (req, res) => {
  res.send('Hi');
};

💡 Hint

Check the arrow function syntax and parentheses.

The correct syntax uses functions.https.onRequest((req, res) => { ... }) with parentheses around parameters and the arrow function inside.

❓ optimization

advanced

Which option optimizes response time for a Firebase HTTP trigger function?

You want your Firebase HTTP trigger function to respond quickly without waiting for asynchronous tasks. Which option achieves this?

Firebase

exports.fastResponse = functions.https.onRequest(async (req, res) => {
  // Some async task
  await someAsyncTask();
  res.send('Done');
});

A Remove <code>await</code> and call <code>someAsyncTask()</code> without waiting, then send response immediately.

B Use <code>setTimeout</code> to delay the response by 2 seconds.

C Keep <code>await</code> so the function waits for the async task before responding.

D Return a promise that never resolves to keep the function alive.

💡 Hint

Think about sending the response without waiting for the async task.

Removing await lets the function send the response immediately, improving response time. The async task runs in the background.

🧠 Conceptual

advanced

What happens if a Firebase HTTP trigger function does not send a response?

If a Firebase HTTP trigger function does not call response.send() or similar methods, what will happen?

A The function logs an error but sends a 200 OK response.

B Firebase automatically sends a default success response.

C The client request will hang until timeout because no response is sent.

D The function throws a syntax error immediately.

💡 Hint

Think about what happens when a server never replies to a request.

If no response is sent, the client waits until the request times out, causing a hanging request.

🔧 Debug

expert

Why does this Firebase HTTP trigger function cause a timeout error?

Examine this function:

exports.timeoutFunc = functions.https.onRequest((req, res) => {
  someAsyncOperation();
  // Missing response.send or end
});

Why does this cause a timeout error?

A Because the function never sends a response, the client waits indefinitely until timeout.

B Because <code>someAsyncOperation()</code> is not awaited, it causes a syntax error.

C Because <code>response.send()</code> is called twice causing a conflict.

D Because the function returns a promise that rejects immediately.

💡 Hint

Check if the function sends any response back to the client.

The function calls an async operation but never sends a response. This causes the client to wait until timeout.

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()