Default value pipe in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the DefaultValuePipe from NestJS.

NestJS

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

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

AValidationPipe

BParseIntPipe

CDefaultValuePipe

DUsePipes

Attempts:

3 left

💡 Hint

Common Mistakes

Importing the wrong pipe like ValidationPipe or ParseIntPipe.

Forgetting to import the pipe.

The DefaultValuePipe is imported from '@nestjs/common' to provide default values for parameters.

2fill in blank

medium

Complete the code to apply DefaultValuePipe with a default value of 'guest' to the 'username' query parameter.

NestJS

@Query('username', new [1]('guest')) username: string

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

ADefaultValuePipe

BValidationPipe

CParseBoolPipe

DParseIntPipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using ParseIntPipe or ValidationPipe instead of DefaultValuePipe.

Not passing the default value as a constructor argument.

DefaultValuePipe is used to provide a default value when the query parameter is missing.

3fill in blank

hard

Fix the error in the code to correctly use DefaultValuePipe with a default value of 10 for the 'limit' query parameter.

NestJS

@Query('limit', new [1](10)) limit: number

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

AParseBoolPipe

BValidationPipe

CParseArrayPipe

DDefaultValuePipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using pipes that do not accept default values like ParseBoolPipe.

Not wrapping the pipe with 'new'.

DefaultValuePipe accepts a default value and sets it if the query parameter is missing.

4fill in blank

hard

Fill both blanks to create a controller method that uses DefaultValuePipe to set a default page number of 1 and a default page size of 20.

NestJS

getItems(@Query('page', new [1](1)) page: number, @Query('size', new [2](20)) size: number) { return { page, size }; }

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

ADefaultValuePipe

BParseIntPipe

CValidationPipe

DParseBoolPipe

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing different pipes for the two parameters.

Using pipes that do not accept default values.

DefaultValuePipe is used for both 'page' and 'size' to provide default values when parameters are missing.

5fill in blank

hard

Fill all three blanks to create a controller method that uses DefaultValuePipe and ParseIntPipe to set default values and parse integers for 'page', 'size', and 'sortOrder' query parameters.

NestJS

getData(@Query('page', new [1](1), new [2]()) page: number, @Query('size', new [3](10), new ParseIntPipe()) size: number, @Query('sortOrder', new DefaultValuePipe('asc')) sortOrder: string) { return { page, size, sortOrder }; }

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

ADefaultValuePipe

BParseIntPipe

CValidationPipe

DParseBoolPipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using ParseIntPipe before DefaultValuePipe.

Not using DefaultValuePipe for default values.

Using ValidationPipe instead of ParseIntPipe.

DefaultValuePipe sets default values; ParseIntPipe parses string to integer. The method uses both pipes for 'page' and 'size'.

Practice

1. What is the main purpose of the DefaultValuePipe in NestJS?

easy

A. To provide a fallback value when a request parameter is missing

B. To validate the type of a request parameter

C. To transform a string into a number

D. To throw an error if a parameter is missing

Solution

1. Step 1: Understand the role of DefaultValuePipe

   The DefaultValuePipe is used to supply a default value when the expected input is not provided in the request.

2. Step 2: Compare with other pipe functions

   Unlike validation or transformation pipes, DefaultValuePipe does not validate or convert but only sets a fallback value.

3. Final Answer:

   To provide a fallback value when a request parameter is missing -> Option A

4. Quick Check:

   DefaultValuePipe = fallback value [OK]

Hint: DefaultValuePipe sets fallback for missing inputs [OK]

Common Mistakes:

• Confusing DefaultValuePipe with validation pipes
• Thinking it throws errors on missing params
• Assuming it transforms data types

2. Which of the following is the correct way to use DefaultValuePipe to set a default value of 10 for a query parameter named page?

easy

A. @Query('page', new DefaultValuePipe(10)) page: number

B. @Query('page', DefaultValuePipe(10)) page: number

C. @Query('page', DefaultValuePipe) page: number = 10

D. @Query('page', new DefaultValuePipe) page: number = 10

Solution

1. Step 1: Check the syntax for using pipes in NestJS

   Pipes are instantiated with the new keyword and passed as the second argument to decorators like @Query.

2. Step 2: Confirm the correct usage of DefaultValuePipe

   The DefaultValuePipe requires a value in its constructor, so new DefaultValuePipe(10) is correct.

3. Final Answer:

   @Query('page', new DefaultValuePipe(10)) page: number -> Option A

4. Quick Check:

   Use new DefaultValuePipe(value) with decorator [OK]

Hint: Always use 'new' to instantiate DefaultValuePipe [OK]

Common Mistakes:

• Omitting 'new' keyword when creating the pipe
• Passing DefaultValuePipe without parentheses
• Assigning default value directly in parameter without pipe

3. Given the following controller method, what will be the value of limit if the request URL is /items (no query parameters)?

@Get('items')
getItems(@Query('limit', new DefaultValuePipe(5)) limit: number) {
  return limit;
}

medium

A. undefined

B. null

C. 5

D. 0

Solution

1. Step 1: Analyze the request URL and query parameters

   The URL /items has no limit query parameter provided.

2. Step 2: Understand DefaultValuePipe behavior

   Since limit is missing, DefaultValuePipe sets it to the default value 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Missing param uses default value 5 [OK]

Hint: Missing query uses DefaultValuePipe value [OK]

Common Mistakes:

• Assuming undefined when param is missing
• Confusing null with default value
• Expecting zero instead of default

4. Identify the error in this NestJS controller method using DefaultValuePipe:

@Get('users')
getUsers(@Query('page', DefaultValuePipe(1)) page: number) {
  return page;
}

medium

A. Default value must be a string, not a number

B. DefaultValuePipe cannot be used with @Query decorator

C. Parameter 'page' should be optional with '?'

D. Missing 'new' keyword when instantiating DefaultValuePipe

Solution

1. Step 1: Check how DefaultValuePipe is instantiated

   The pipe must be created with the new keyword, but here it is called like a function.

2. Step 2: Confirm correct usage with @Query

   DefaultValuePipe works with @Query, so no error there.

3. Final Answer:

   Missing 'new' keyword when instantiating DefaultValuePipe -> Option D

4. Quick Check:

   Always use 'new' with DefaultValuePipe [OK]

Hint: Remember 'new' before DefaultValuePipe instance [OK]

Common Mistakes:

• Calling DefaultValuePipe as a function without 'new'
• Thinking DefaultValuePipe only works with @Param
• Making parameter optional instead of using pipe

5. You want to create a controller method that accepts an optional sort query parameter with a default value of 'asc'. Additionally, if the client sends an empty string for sort, it should also default to 'asc'. Which approach correctly achieves this behavior?

hard

A. Use @Query('sort', new DefaultValuePipe('asc')) sort: string

B. Use a custom pipe that returns 'asc' if value is missing or empty string

C. Use @Query('sort', new DefaultValuePipe('asc')) sort: string = 'asc'

D. Use @Query('sort') sort: string = 'asc' without any pipe

Solution

1. Step 1: Understand DefaultValuePipe limitations

   DefaultValuePipe only sets default if value is missing (undefined), not if empty string is provided.

2. Step 2: Consider handling empty string case

   To treat empty string as missing, a custom pipe is needed to check both missing and empty values and return 'asc'.

3. Step 3: Evaluate other options

   Use @Query('sort', new DefaultValuePipe('asc')) sort: string = 'asc' sets default but does not handle empty string; Use @Query('sort', new DefaultValuePipe('asc')) sort: string does not handle empty string; Use @Query('sort') sort: string = 'asc' without any pipe ignores pipe usage.

4. Final Answer:

   Use a custom pipe that returns 'asc' if value is missing or empty string -> Option B

5. Quick Check:

   DefaultValuePipe ignores empty string, custom pipe needed [OK]

Hint: DefaultValuePipe skips empty strings; use custom pipe [OK]

Common Mistakes:

• Assuming DefaultValuePipe handles empty strings
• Setting default in parameter but ignoring pipe
• Not handling empty string case explicitly