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.