Query parameters in NestJS - Step-by-Step Execution

Concept Flow - Query parameters

Client sends HTTP GET request

↓

NestJS Controller receives request

↓

Extract query parameters using @Query()

↓

Use query parameters in handler logic

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Send response back to client

The flow shows how a NestJS controller extracts query parameters from an HTTP GET request and uses them in the handler.

Execution Sample

NestJS

import { Controller, Get, Query } from '@nestjs/common';

@Controller('items')
export class ItemsController {
  @Get()
  findAll(@Query('search') search: string) {
    return `Searching for: ${search}`;
  }
}

This code defines a GET endpoint that reads the 'search' query parameter and returns a message including it.

Execution Table

Step	Action	Request URL	Query Extracted	Handler Output
1	Client sends GET request	/items?search=book	N/A	N/A
2	NestJS receives request	/items?search=book	search=book	N/A
3	Controller method called	/items?search=book	search=book	N/A
4	Return response	/items?search=book	search=book	Searching for: book
5	Response sent to client	/items?search=book	search=book	Searching for: book

💡 Request handled and response sent back to client with query parameter value.

Variable Tracker

Variable	Start	After Request
search	undefined	"book"

Key Moments - 2 Insights

Why do we use @Query('search') instead of just @Query()?

Using @Query('search') extracts only the 'search' parameter from the query string, as shown in execution_table step 3. Using @Query() alone would give all query parameters as an object.

What happens if the query parameter is missing?

If 'search' is missing, the variable 'search' will be undefined (see variable_tracker start value). The handler can check for this and handle it accordingly.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of the 'search' query parameter extracted?

A "items"

B "book"

C undefined

D "search"

💡 Hint

Check the 'Query Extracted' column at step 2 in the execution_table.

At which step does the controller method receive the query parameter value?

A Step 3

B Step 1

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column to find when the controller method is called.

If the URL was /items without any query, what would be the value of 'search' in variable_tracker?

A "" (empty string)

B null

C undefined

D 0

💡 Hint

Refer to variable_tracker start value for 'search' when no query parameter is provided.

Concept Snapshot

NestJS Query Parameters:
- Use @Query('paramName') to get a single query parameter.
- The parameter is extracted from the URL after '?'.
- If missing, the value is undefined.
- Use in GET handlers to customize responses.
- Example: /items?search=book extracts 'book' as search.

Full Transcript

In NestJS, query parameters come from the URL after the question mark. The controller uses the @Query decorator to get these values. For example, @Query('search') gets the 'search' parameter. When a client sends a GET request like /items?search=book, NestJS extracts 'book' and passes it to the handler. If the parameter is missing, the value is undefined. The handler can then use this value to customize the response. This flow helps build flexible APIs that respond to client queries.