Query parameters in NestJS - Performance & Optimization

Performance: Query parameters

MEDIUM IMPACT

Query parameters affect how the server processes requests and can impact response time and payload size, indirectly influencing page load speed.

Handling user filters via query parameters in a NestJS API

NestJS

import { Controller, Get, Query } from '@nestjs/common';
import { IsOptional, IsString, IsInt } from 'class-validator';

class ItemQueryDto {
  @IsOptional()
  @IsString()
  category?: string;

  @IsOptional()
  @IsInt()
  limit?: number;
}

@Controller('items')
export class ItemsController {
  @Get()
  getItems(@Query() query: ItemQueryDto) {
    // Validated and typed query parameters
    return this.itemsService.find(query);
  }
}

Validating and typing query parameters reduces unnecessary data processing and prevents large, malformed queries, improving server response time.

📈 Performance GainReduces server processing time by 30-50% on average and lowers payload size, improving LCP.

Handling user filters via query parameters in a NestJS API

NestJS

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

@Controller('items')
export class ItemsController {
  @Get()
  getItems(@Query() query: any) {
    // Accepts all query params without validation
    return this.itemsService.find(query);
  }
}

Accepting all query parameters without validation can lead to large, unoptimized queries causing slow database responses and increased payload size.

📉 Performance CostIncreases server processing time and response size, potentially blocking rendering for 100+ ms on complex queries.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Unvalidated large query params	N/A (server-side)	N/A	Increases due to delayed response	[X] Bad
Validated, minimal query params	N/A (server-side)	N/A	Lower due to faster response	[OK] Good

Rendering Pipeline

Query parameters are parsed by the server before generating the response. Large or complex queries increase server processing time, delaying the HTML response and thus the browser's rendering start.

→Server Processing

→Network Transfer

→Rendering Start

⚠️ BottleneckServer Processing

Core Web Vital Affected

LCP

Query parameters affect how the server processes requests and can impact response time and payload size, indirectly influencing page load speed.

Optimization Tips

1Validate and type query parameters to avoid unnecessary server processing.

2Keep query parameters minimal and relevant to reduce payload size.

3Avoid sending large or complex data in query parameters to improve response time.

Performance Quiz - 3 Questions

How do large, unvalidated query parameters affect page load performance?

A They reduce the size of the HTML response.

B They increase server processing time and delay content rendering.

C They improve browser caching efficiency.

D They have no impact on performance.

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by XHR or Fetch, inspect request URLs and response times for requests with query parameters.

What to look for: Look for long server response times and large payload sizes indicating inefficient query parameter usage.

Practice

1. What is the purpose of using @Query() decorator in a NestJS controller?

easy

A. To inject a service into the controller

B. To define a new route path

C. To handle POST request bodies

D. To access query parameters sent in the URL

Solution

1. Step 1: Understand the role of @Query()

   The @Query() decorator extracts query parameters from the URL in a GET request.

2. Step 2: Compare with other decorators

   Other decorators like @Body() handle POST bodies, and @Param() handles route params, so they don't fit here.

3. Final Answer:

   To access query parameters sent in the URL -> Option D

4. Quick Check:

   @Query() reads URL queries = A [OK]

Hint: Remember: @Query() reads URL query strings [OK]

Common Mistakes:

• Confusing @Query() with @Body()
• Thinking @Query() defines routes
• Using @Query() to inject services

2. Which of the following is the correct syntax to get a query parameter named page in a NestJS controller method?

easy

A. async getItems(@Param('page') page: number) {}

B. async getItems(@Body('page') page: number) {}

C. async getItems(@Query('page') page: number) {}

D. async getItems(@Query.page page: number) {}

Solution

1. Step 1: Identify correct decorator usage

   To get a specific query parameter, use @Query('paramName') syntax.

2. Step 2: Check syntax correctness

   async getItems(@Query('page') page: number) {} uses @Query('page') correctly. Options A and C use wrong decorators, and D has invalid syntax.

3. Final Answer:

   async getItems(@Query('page') page: number) {} -> Option C

4. Quick Check:

   Use @Query('param') for query params = B [OK]

Hint: Use @Query('name') to get specific query param [OK]

Common Mistakes:

• Using @Param() instead of @Query()
• Trying to access query param as @Query.page
• Using @Body() for query parameters

3. Given this NestJS controller method:

async find(@Query() query) {
  return query.page + '-' + query.limit;
}

What will be the output if the URL is /items?page=2&limit=5?

medium

A. "2-5"

B. "page-limit"

C. "undefined-undefined"

D. Runtime error

Solution

1. Step 1: Understand @Query() behavior

   The @Query() decorator without arguments returns an object with all query parameters as strings.

2. Step 2: Check the returned string concatenation

   With URL ?page=2&limit=5, query.page is "2" and query.limit is "5". Concatenation returns "2-5".

3. Final Answer:

   "2-5" -> Option A

4. Quick Check:

   Query object has keys = "2-5" [OK]

Hint: Without param name, @Query() gets all query params as object [OK]

Common Mistakes:

• Expecting numbers instead of strings from query params
• Assuming @Query() returns undefined
• Thinking it causes runtime error

4. What is wrong with this NestJS controller method to get a query parameter search?:

async search(@Query(search) searchTerm: string) {
  return searchTerm;
}

medium

A. The method must be synchronous, not async

B. The @Query decorator argument should be a string: @Query('search')

C. The parameter name cannot be searchTerm

D. The method should use @Param('search') instead

Solution

1. Step 1: Check @Query decorator syntax

   The argument to @Query() must be a string literal naming the query parameter, e.g., @Query('search').

2. Step 2: Identify the error in the code

   The code uses @Query(search) without quotes, which is invalid syntax and causes an error.

3. Final Answer:

   The @Query decorator argument should be a string: @Query('search') -> Option B

4. Quick Check:

   Decorator argument must be string literal = D [OK]

Hint: Always quote query param name in @Query('name') [OK]

Common Mistakes:

• Forgetting quotes around query param name
• Using @Param() for query params
• Thinking async keyword is disallowed

5. You want to create a NestJS GET endpoint that accepts optional query parameters sort and filter. If filter is missing, it should default to "all". Which code snippet correctly implements this behavior?

hard

A. async getItems(@Query() query) {
const filter = query.filter || 'all';
return { sort: query.sort, filter };
}

B. async getItems(@Query('sort') sort: string, @Query('filter') filter: string = 'all') {}

C. async getItems(@Query('sort') sort: string, @Query('filter') filter?: string) {
if (!filter) filter = 'all';
return { sort, filter };
}

D. async getItems(@Query('sort') sort: string, @Query('filter') filter: string) {
filter = filter ?? 'all';
return { sort, filter };
}

Solution

1. Step 1: Understand default value handling for query params

   Query parameters come as strings or undefined. Default values should be set inside the method, not in parameter defaults.

2. Step 2: Analyze each option

   async getItems(@Query('sort') sort: string, @Query('filter') filter: string = 'all') {} tries to set default in parameter, which doesn't work well with @Query(). async getItems(@Query() query) { const filter = query.filter || 'all'; return { sort: query.sort, filter }; } extracts all query params and sets default for filter inside method correctly. async getItems(@Query('sort') sort: string, @Query('filter') filter?: string) { if (!filter) filter = 'all'; return { sort, filter }; } tries to assign to parameter which is readonly. async getItems(@Query('sort') sort: string, @Query('filter') filter: string) { filter = filter ?? 'all'; return { sort, filter }; } assigns default after parameter, but parameter is readonly, so it causes error.

3. Final Answer:

   async getItems(@Query() query) { const filter = query.filter || 'all'; return { sort: query.sort, filter }; } -> Option A

4. Quick Check:

   Set defaults inside method body = A [OK]

Hint: Set default query values inside method, not in parameters [OK]

Common Mistakes:

• Trying to assign default values in parameter list with @Query()
• Mutating readonly parameters
• Not handling missing query params properly