Default value pipe in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when using DefaultValuePipe with a missing query parameter?

Consider this NestJS controller method:

  @Get('items')
  getItems(@Query('limit', new DefaultValuePipe('10')) limit: string) {
    return limit;
  }

If a client calls /items without the limit query parameter, what will be the response?

A "10"

B undefined

C null

D 0

💡 Hint

Think about what DefaultValuePipe does when the parameter is missing.

The DefaultValuePipe provides a default value when the parameter is missing or undefined. Since limit is missing, it returns the default string "10".

📝 Syntax

intermediate

Which option correctly applies DefaultValuePipe to a route parameter?

Given this NestJS route handler, which option correctly applies DefaultValuePipe to the id route parameter with a default of '42'?

  @Get(':id')
  getById( /* what goes here? */ ) {
    return id;
  }

A @Param('id', new DefaultValuePipe(42)) id: number

B @Param('id', new DefaultValuePipe('42')) id: string

C @Param('id', DefaultValuePipe('42')) id: string

D @Param('id', new DefaultValuePipe) id: string = '42'

💡 Hint

Remember how to instantiate pipes and pass default values.

Option B correctly creates a new DefaultValuePipe instance with the string '42' and applies it to the 'id' parameter.

🔧 Debug

advanced

Why does this DefaultValuePipe usage cause a runtime error?

Examine this code snippet:

  @Get()
  getData(@Query('page', new DefaultValuePipe(1)) page: number) {
    return page + 1;
  }

When calling /endpoint without the page query parameter, a runtime error occurs. Why?

A The query parameter 'page' is required and cannot have a default.

B DefaultValuePipe cannot accept numbers as default values, only strings.

C The pipe is not instantiated correctly; it needs parentheses after the class name.

D DefaultValuePipe returns a string '1', but page is typed as number, causing a type error when adding.

💡 Hint

Consider the type of the default value returned by DefaultValuePipe.

DefaultValuePipe returns the default value as-is, which is a number here, but query parameters are strings by default. The method expects a number, but the addition operation fails because the value is a string, causing a runtime error.

🧠 Conceptual

advanced

What happens if DefaultValuePipe is combined with ParseIntPipe on a query parameter?

Consider this NestJS method:

  @Get()
  getItems(
    @Query('limit', new DefaultValuePipe('5'), new ParseIntPipe()) limit: number
  ) {
    return limit * 2;
  }

If the client calls /items without the limit parameter, what will be the output?

A 10

B NaN

C 5

D Error thrown by ParseIntPipe

💡 Hint

Think about the order pipes run and how ParseIntPipe converts strings.

DefaultValuePipe sets '5' as a string default. ParseIntPipe converts '5' to number 5. The method returns 5 * 2 = 10.

❓ state_output

expert

What is the value of 'offset' after this NestJS controller method runs?

Given this method:

  @Get()
  getOffset(
    @Query('offset', new DefaultValuePipe('0'), new ParseIntPipe()) offset: number
  ) {
    return offset;
  }

If the client calls /endpoint?offset=abc, what happens?

A offset is 0 because DefaultValuePipe sets default and ParseIntPipe converts it

B offset is NaN because 'abc' cannot be parsed to number

C ParseIntPipe throws a BadRequestException due to invalid number

D offset is the string 'abc' because DefaultValuePipe overrides

💡 Hint

What does ParseIntPipe do when it cannot parse a string to a number?

ParseIntPipe throws a BadRequestException if the input is not a valid number. Since 'abc' is invalid, the request fails with an error.

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