Response handling in NestJS - Step-by-Step Execution

Concept Flow - Response handling

Receive HTTP Request

↓

Controller Method Called

↓

Process Request Logic

↓

Prepare Response Data

↓

Send Response to Client

↓

Client Receives Response

This flow shows how NestJS handles an HTTP request by calling a controller, processing logic, preparing data, and sending the response back to the client.

Execution Sample

NestJS

import { Controller, Get, Res } from '@nestjs/common';
import { Response } from 'express';

@Controller('hello')
export class HelloController {
  @Get()
  getHello(@Res() res: Response) {
    res.status(200).json({ message: 'Hello World' });
  }
}

This code defines a NestJS controller that sends a JSON response with status 200 and a message when the /hello route is requested.

Execution Table

Step	Action	Input/State	Output/State Change	Notes
1	Receive HTTP GET /hello	Request received	Controller method getHello called	NestJS routes request to controller
2	Call getHello method	No parameters except response object	Prepare response with status 200 and JSON body	Uses Express Response object
3	Execute res.status(200).json(...)	Response object	HTTP response sent with status 200 and JSON { message: 'Hello World' }	Response sent to client
4	Client receives response	HTTP 200 with JSON body	Client can read message 'Hello World'	End of request-response cycle

💡 Response sent to client, request handling complete

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
res	Express Response object	Response status set to 200	Response body set to JSON { message: 'Hello World' }	Response sent to client

Key Moments - 2 Insights

Why do we use @Res() to get the response object instead of just returning data?

Using @Res() gives full control over the response, like setting status codes and headers manually, as shown in step 2 and 3 of the execution_table.

What happens if we forget to call res.status(...).json(...) inside the controller?

The client will not receive a response and the request will hang, because the response is never sent. This is clear from step 3 where sending the response ends the cycle.

Visual Quiz - 3 Questions

Look at the execution table, what is the response status code set at step 3?

A 500

B 404

C 200

D 302

💡 Hint

Check the 'Output/State Change' column at step 3 in the execution_table.

At which step is the controller method getHello called?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column to find when getHello is executed.

If we remove @Res() and just return { message: 'Hello World' }, what changes in the response handling?

A NestJS automatically sends JSON with status 200

B Response status must be set manually

C Response will not be sent

D Client receives plain text instead of JSON

💡 Hint

Consider how NestJS handles return values when @Res() is not used.

Concept Snapshot

NestJS Response Handling:
- Use @Res() to access Express response object
- Call res.status(code).json(data) to send JSON with status
- Without @Res(), return value is auto-sent as JSON with status 200
- Response ends request cycle
- Control response headers and status manually with @Res()

Full Transcript

In NestJS, when a client sends an HTTP request, the framework routes it to the matching controller method. Inside the method, you can use the @Res() decorator to get the Express response object. This lets you set the HTTP status code and send JSON data manually using res.status(200).json({ message: 'Hello World' }). This sends the response back to the client and ends the request cycle. If you do not use @Res(), you can simply return an object and NestJS will automatically send it as JSON with status 200. The execution table shows each step from receiving the request, calling the controller, preparing the response, and sending it. The variable tracker shows how the response object changes as status and body are set. Key moments clarify why @Res() is used and what happens if the response is not sent. The quiz tests understanding of status codes, method calls, and automatic response sending.