Request and Response objects in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Deno server response?

Consider this Deno server code snippet. What will the client receive as the response body?

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

serve((req) => {
  return new Response("Hello from Deno!", {
    status: 200,
    headers: { "content-type": "text/plain" },
  });
});

A "Hello from Deno!" with status 200 and content-type text/plain

B Empty response with status 404

C "Hello from Deno!" but with status 500

D Response with JSON content-type and body { message: "Hello from Deno!" }

💡 Hint

Look at the Response constructor parameters: body, status, and headers.

The Response object is created with a string body "Hello from Deno!", status 200, and header content-type set to text/plain. This means the client receives exactly that text with a success status.

❓ state_output

intermediate

What is the value of the request method in this handler?

Given this Deno request handler, what will be the value of the variable `method`?

Deno

import { serve } from "https://deno.land/std@0.177.0/http/server.ts";

serve((req) => {
  const method = req.method;
  return new Response(`Method used: ${method}`);
});

A "GET" if the client sends a GET request

B "POST" regardless of client request method

C Always "HEAD"

D Undefined, because req.method does not exist

💡 Hint

Check the Request object properties in Deno's HTTP server.

The Request object has a property `method` that reflects the HTTP method used by the client, such as GET, POST, etc. So if the client sends GET, method will be "GET".

📝 Syntax

advanced

Which option correctly reads JSON from a Deno Request object?

You want to parse JSON data sent in the body of a POST request in Deno. Which code snippet correctly does this?

Deno

async function handler(req) {
  // parse JSON body here
}

A const data = req.body.json();

B const data = JSON.parse(req.body);

C const data = await JSON.parse(req.text());

D const data = await req.json();

💡 Hint

The Request object has a method to parse JSON asynchronously.

The Request object in Deno has an async method `.json()` that returns the parsed JSON body. Other options either misuse synchronous parsing or incorrect method calls.

🔧 Debug

advanced

Why does this Deno response cause a runtime error?

Identify the error in this response creation code snippet:

Deno

return new Response({ message: "Hi" }, { status: 200 });

A Response constructor requires a Promise as the first argument

B Response body must be a string, ReadableStream, or Uint8Array, not an object

C Headers object is required but missing

D Missing status code in Response options

💡 Hint

Check the allowed types for the Response body parameter.

The Response constructor expects the body to be a string, stream, or binary data. Passing a plain object causes a runtime TypeError.

🧠 Conceptual

expert

How does Deno's Request object handle URL query parameters?

Given a request to URL `https://example.com/api?search=deno&limit=5`, how can you access the value of `limit` from the Request object in Deno?

Deno

async function handler(req) {
  // Access query parameters here
}

A const limit = req.query.limit;

B const limit = req.params.limit;

C const url = new URL(req.url); const limit = url.searchParams.get('limit');

D const limit = req.get('limit');

💡 Hint

The Request object has a url string property you can parse with the URL class.

Deno's Request object has a `url` string property. You can create a new URL object from it and then use `searchParams.get()` to access query parameters.

Practice

1. What does the Request object represent in a Deno web server?

easy

A. The data sent by the server to the user

B. The server's file system access

C. The server configuration settings

D. The data sent by the user to the server

Solution

1. Step 1: Understand the role of Request object

   The Request object contains information about what the user sends to the server, such as headers, method, and body.

2. Step 2: Differentiate from Response object

   The Response object is what the server sends back, so it is not the Request.

3. Final Answer:

   The data sent by the user to the server -> Option D

4. Quick Check:

   Request = user data [OK]

Hint: Request = user input; Response = server output [OK]

Common Mistakes:

• Confusing Request with Response
• Thinking Request holds server settings
• Assuming Request is server output

2. Which of the following is the correct way to create a Response object with a JSON body in Deno?

easy

A. Response.json(data)

B. new Response(data, { type: 'json' })

C. new Response(data, { headers: { 'Content-Type': 'application/json' } })

D. new Response(data, { contentType: 'application/json' })

Solution

1. Step 1: Check how to send JSON in Response

   In Deno, you can create a Response with JSON by stringifying the data and setting the 'Content-Type' header to 'application/json'.

2. Step 2: Validate options

   new Response(data, { headers: { 'Content-Type': 'application/json' } }) correctly sets the header and passes data (assuming data is stringified). Response.json(data) is invalid because Response.json() is not a standard method in Deno's Response API. Options B and D use incorrect properties.

3. Final Answer:

   new Response(data, { headers: { 'Content-Type': 'application/json' } }) -> Option C

4. Quick Check:

   Response with JSON header [OK]

Hint: Use new Response(JSON.stringify(data), { headers: { 'Content-Type': 'application/json' } }) [OK]

Common Mistakes:

• Passing raw object without stringify
• Using incorrect init properties like 'type' or 'contentType'
• Assuming Response.json() exists

3. Given the following Deno server code snippet, what will be the response body when a user sends a GET request?

const handler = (req) => {
  if (req.method === 'GET') {
    return new Response('Hello World');
  }
  return new Response('Method Not Allowed', { status: 405 });
};

medium

A. "Method Not Allowed"

B. "Hello World"

C. Empty response

D. Error thrown

Solution

1. Step 1: Check request method condition

   The code checks if the request method is 'GET'. If yes, it returns 'Hello World'.

2. Step 2: Determine response for GET request

   Since the user sends a GET request, the first condition matches and returns 'Hello World'.

3. Final Answer:

   "Hello World" -> Option B

4. Quick Check:

   GET request returns 'Hello World' [OK]

Hint: Check req.method to decide response content [OK]

Common Mistakes:

• Ignoring method check and picking wrong response
• Assuming default response is empty
• Thinking error is thrown on GET

4. Identify the error in this Deno request handler code:

const handler = (req) => {
  if (req.method = 'POST') {
    return new Response('Posted');
  }
  return new Response('Not Posted');
};

medium

A. Using assignment (=) instead of comparison (===) in if condition

B. Missing await keyword before Response

C. Response body must be JSON, not string

D. req.method is undefined

Solution

1. Step 1: Check the if condition syntax

   The code uses single equals (=) which assigns 'POST' to req.method instead of comparing it.

2. Step 2: Understand correct comparison operator

   To compare, use '===' or '==' instead of '=' to avoid bugs.

3. Final Answer:

   Using assignment (=) instead of comparison (===) in if condition -> Option A

4. Quick Check:

   Use '===' for comparison, not '=' [OK]

Hint: Use '===' for comparisons, '=' is assignment [OK]

Common Mistakes:

• Confusing assignment and comparison operators
• Thinking Response needs await here
• Assuming req.method is missing

5. How can you read JSON data sent in the body of a POST request using the Deno Request object?

hard

A. Use await req.json() to parse the JSON body

B. Use req.body.json() without await

C. Use JSON.parse(req.body) directly

D. Use req.getJson() method

Solution

1. Step 1: Understand how to parse JSON body in Deno

   The Request object has an async method json() that returns the parsed JSON from the body.

2. Step 2: Confirm correct usage

   You must await req.json() because it returns a Promise resolving to the parsed object.

3. Final Answer:

   Use await req.json() to parse the JSON body -> Option A

4. Quick Check:

   await req.json() parses JSON body correctly [OK]

Hint: Always await req.json() to get JSON body [OK]

Common Mistakes:

• Calling json() without await
• Trying to parse req.body directly
• Using non-existent getJson() method