Request and Response objects in Deno

Introduction

Request and Response objects let your server talk with users. The Request holds what the user sends. The Response is what your server sends back.

When you want to get data a user sends to your server, like form info or URL details.

When you need to send back a message, webpage, or data to the user.

When building APIs that receive requests and return JSON or other data.

When handling different types of requests like GET, POST, or PUT.

When you want to control headers, status codes, or cookies in your server replies.

Syntax

Deno

async function handler(request: Request): Promise<Response> {
  // Use request to read user data
  // Create and return a Response
  return new Response('Hello world', { status: 200 });
}

The Request object contains info about the user's request like method, headers, and body.

The Response object is what you send back. You create it with content, status, and headers.

Examples

Reads the URL path from the request and sends it back in the response.

Deno

const handler = async (request: Request): Promise<Response> => {
  const url = new URL(request.url);
  return new Response(`You asked for: ${url.pathname}`);
};

Checks if the request is POST, reads JSON data, and replies with it.

Deno

const handler = async (request: Request): Promise<Response> => {
  if (request.method === 'POST') {
    const data = await request.json();
    return new Response(`Got data: ${JSON.stringify(data)}`);
  }
  return new Response('Send a POST request');
};

Sends a 404 Not Found response without reading the request body.

Deno

const handler = (request: Request): Response => {
  return new Response('Not Found', { status: 404 });
};

Sample Program

This Deno server listens on port 8000. It handles GET requests to /hello by sending a greeting. It handles POST requests to /data by reading JSON data and replying with it. Other requests get a 404 response.

Deno

import { serve } from "https://deno.land/std@0.201.0/http/server.ts";

const handler = async (request: Request): Promise<Response> => {
  const { pathname } = new URL(request.url);

  if (request.method === "GET" && pathname === "/hello") {
    return new Response("Hello from Deno!", {
      status: 200,
      headers: { "content-type": "text/plain; charset=utf-8" },
    });
  }

  if (request.method === "POST" && pathname === "/data") {
    try {
      const data = await request.json();
      return new Response(`Received data: ${JSON.stringify(data)}`, {
        status: 200,
        headers: { "content-type": "application/json" },
      });
    } catch {
      return new Response("Invalid JSON", { status: 400 });
    }
  }

  return new Response("Not Found", { status: 404 });
};

console.log("Server running on http://localhost:8000");
serve(handler, { port: 8000 });

Important Notes

Always check the request method (GET, POST, etc.) before reading the body.

Use await request.json() to read JSON data safely.

Set proper content-type headers in your Response for better browser handling.

Summary

Request objects hold what the user sends to your server.

Response objects are what your server sends back to the user.

Use these objects to build interactive and data-driven web servers in Deno.

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