Request body in NestJS - Step-by-Step Execution

Concept Flow - Request body

Client sends HTTP request

↓

NestJS Controller receives request

↓

@Body() decorator extracts body data

↓

Controller method uses extracted data

↓

Response sent back to client

The flow shows how NestJS extracts the request body using @Body() in a controller method to use the data.

Execution Sample

NestJS

import { Controller, Post, Body } from '@nestjs/common';

@Controller('users')
export class UsersController {
  @Post()
  createUser(@Body() body: any) {
    return body;
  }
}

This code defines a POST endpoint '/users' that receives JSON data in the request body and returns it.

Execution Table

Step	Action	Input	Body Extracted	Output
1	Client sends POST /users	{"name":"Alice"}	N/A	N/A
2	NestJS receives request	POST /users with body	N/A	N/A
3	@Body() extracts body	{"name":"Alice"}	{"name":"Alice"}	N/A
4	Controller method runs	{"name":"Alice"}	{"name":"Alice"}	{"name":"Alice"}
5	Response sent	N/A	N/A	{"name":"Alice"}

💡 Request handled and response sent with extracted body data.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
body	undefined	{"name":"Alice"}	{"name":"Alice"}	{"name":"Alice"}

Key Moments - 2 Insights

Why do we use @Body() in the controller method?

The @Body() decorator tells NestJS to extract the JSON data sent in the request body so the method can use it, as shown in step 3 of the execution_table.

What happens if the client sends no body?

Then @Body() extracts undefined or an empty object, so the variable 'body' will be empty, which you can check before using it (not shown in this trace).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'body' after step 3?

A undefined

B {"name":"Alice"}

C null

D {}

💡 Hint

Check the 'Body Extracted' column at step 3 in the execution_table.

At which step does the controller method return the extracted body?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Output' column in the execution_table to see when the body is returned.

If the client sends an empty body, how would the 'body' variable change in variable_tracker?

A It would cause an error and stop execution

B It would be the same as before step 3

C It would be undefined or empty object after step 3

D It would contain previous request data

💡 Hint

Refer to key_moments about empty body and variable_tracker initial values.

Concept Snapshot

NestJS Request Body Handling:
- Use @Body() decorator in controller method parameter.
- Extracts JSON data sent by client in request body.
- Works with POST, PUT, PATCH requests.
- Access extracted data as method argument.
- Return or process data as needed.

Full Transcript

In NestJS, when a client sends an HTTP request with a body, such as JSON data, the controller can access this data using the @Body() decorator. The flow starts with the client sending a POST request to an endpoint. NestJS receives the request and uses @Body() to extract the body content. The controller method then receives this extracted data as a parameter and can use it, for example, to return it back or save it. If the client sends no body, the extracted data will be undefined or empty. This process allows easy handling of request data in NestJS controllers.