Default value pipe in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Default Value Pipe in NestJS?

The Default Value Pipe sets a default value for a route parameter or query if the client does not provide one. It helps avoid undefined or missing values in handlers.

beginner

How do you apply the Default Value Pipe to a query parameter in NestJS?

You use it in the controller method parameter like this:
@Query('page', new DefaultValuePipe(1)) page: number
This means if 'page' is missing, it defaults to 1.

intermediate

Can the Default Value Pipe be combined with other pipes in NestJS? How?

Yes. You can chain pipes by passing them as separate arguments:
@Query('limit', new DefaultValuePipe(10), new ParseIntPipe()) limit: number
This sets a default and then converts to number.

beginner

What happens if the Default Value Pipe is not used and a query parameter is missing?

The parameter value will be undefined in the controller method, which may cause errors if not handled properly.

beginner

Write a simple example of a NestJS controller method using Default Value Pipe for a 'sort' query parameter with default 'asc'.

  @Get()
  getItems(@Query('sort', new DefaultValuePipe('asc')) sort: string) {
    return `Sorting items by ${sort}`;
  }

What does the Default Value Pipe do in NestJS?

A Blocks requests without parameters

B Sets a default value if the input is missing

C Transforms input to uppercase

D Validates the input format

The Default Value Pipe provides a fallback value when the client does not send a parameter.

How do you apply Default Value Pipe to a route parameter named 'id' with default 0?

A @Param('id', new DefaultValuePipe(0)) id: number

B @Query('id', new DefaultValuePipe(0)) id: number

C @Body('id', new DefaultValuePipe(0)) id: number

D @Param('id', ParseIntPipe) id: number

Route parameters use @Param decorator; DefaultValuePipe sets default if missing.

Which of these is a valid way to chain Default Value Pipe with ParseIntPipe?

A @Query('page', [new DefaultValuePipe(1), new ParseIntPipe()]) page: number

B @Query('page', ParseIntPipe, DefaultValuePipe) page: number

C @Query('page', new DefaultValuePipe(1)) page: number

D @Query('page', new DefaultValuePipe(1), new ParseIntPipe()) page: number

You can pass multiple pipes as separate arguments to the decorator.

If a query parameter is missing and no Default Value Pipe is used, what is the parameter value?

A undefined

B null

C empty string

D default value 0

Missing parameters are undefined unless a default is set.

Which decorator is used to access query parameters in NestJS?

A @Body()

B @Param()

C @Query()

D @Headers()

@Query() accesses query string parameters from the URL.

Explain how the Default Value Pipe improves handling of missing parameters in NestJS controllers.

Think about what happens when a client forgets to send a parameter.

Describe how to combine Default Value Pipe with other pipes like ParseIntPipe in a NestJS controller.

Consider the order of pipes and their roles.

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