Query parameters in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extract query parameters in a NestJS controller method.

NestJS

import { Controller, Get, [1] } from '@nestjs/common';

@Controller('items')
export class ItemsController {
  @Get()
  findAll(@[1]() query: any) {
    return query;
  }
}

Drag options to blanks, or click blank then click option'

AQuery

BBody

CParam

DHeaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Body() instead of @Query() to get query parameters.

Using @Param() which extracts route parameters, not query parameters.

The @Query() decorator extracts query parameters from the URL in NestJS.

2fill in blank

medium

Complete the code to type the query parameter object in a NestJS controller.

NestJS

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

@Controller('search')
export class SearchController {
  @Get()
  search(@Query() query: [1]) {
    return query;
  }
}

Drag options to blanks, or click blank then click option'

Anumber

Bstring

Cany

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using string which only allows a single string, not an object.

Using number or boolean which are too restrictive.

Query parameters can be any shape, so any is often used for flexibility.

3fill in blank

hard

Fix the error in the code to correctly extract a specific query parameter named 'page'.

NestJS

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

@Controller('posts')
export class PostsController {
  @Get()
  getPosts(@Query('[1]') page: string) {
    return `Page number is ${page}`;
  }
}

Drag options to blanks, or click blank then click option'

Apage

BPage

Cpages

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect casing like 'Page' instead of 'page'.

Using plural or different names like 'pages' or 'number'.

The string inside @Query() must exactly match the query parameter name, which is 'page'.

4fill in blank

hard

Fill both blanks to destructure 'limit' and 'offset' query parameters with default values.

NestJS

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

@Controller('data')
export class DataController {
  @Get()
  fetchData(@Query() { [1] = 10, [2] = 0 }: any) {
    return { limit: [1], offset: [2] };
  }
}

Drag options to blanks, or click blank then click option'

Alimit

Boffset

Cpage

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names like 'page' or 'size'.

Not providing default values causing undefined errors.

Destructuring the query object with default values for limit and offset allows setting defaults if parameters are missing.

5fill in blank

hard

Fill all three blanks to validate and transform a query parameter 'count' to a number using a pipe.

NestJS

import { Controller, Get, Query, [1] } from '@nestjs/common';

@Controller('stats')
export class StatsController {
  @Get()
  getStats(@Query('[2]', new [3]()) count: number) {
    return `Count is ${count}`;
  }
}

Drag options to blanks, or click blank then click option'

AParseIntPipe

Bcount

CValidationPipe

DParseBoolPipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using ValidationPipe which does not convert types automatically.

Using ParseBoolPipe which is for boolean values.

Mismatching parameter name.

The ParseIntPipe converts the 'count' query parameter from string to number and validates it.

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