Default value pipe in NestJS

Introduction

The Default value pipe helps set a fallback value when no input is provided. It keeps your app working smoothly without errors.

When a user might skip sending a value in a request.

When you want to ensure a variable always has a value.

When handling optional query parameters in APIs.

When you want to avoid manual checks for undefined or null.

When you want to simplify controller code by setting defaults early.

Syntax

NestJS

new DefaultValuePipe(defaultValue)

The defaultValue is the value used if the input is missing or undefined.

This pipe can be used in controller method parameters to provide default values automatically.

Examples

Sets the default page number to 1 if no page query parameter is sent.

NestJS

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

Defaults the sort order to 'asc' when the sort parameter is missing.

NestJS

@Query('sort', new DefaultValuePipe('asc')) sortOrder: string

Uses 'unknown' as the default id if none is provided in the route.

NestJS

@Param('id', new DefaultValuePipe('unknown')) id: string

Sample Program

This controller method uses DefaultValuePipe to set default values for 'limit' and 'offset' query parameters. If the client does not send these, the defaults 10 and 0 are used.

NestJS

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

@Controller('items')
export class ItemsController {
  @Get()
  getItems(
    @Query('limit', new DefaultValuePipe(10)) limit: number,
    @Query('offset', new DefaultValuePipe(0)) offset: number
  ) {
    return `Limit is ${limit}, Offset is ${offset}`;
  }
}

Important Notes

DefaultValuePipe only sets the value if the input is undefined or missing, not if it is null or empty string.

You can combine DefaultValuePipe with other pipes like ParseIntPipe to validate and convert input.

Summary

DefaultValuePipe sets fallback values for missing inputs.

It simplifies controller code by avoiding manual checks.

Use it with query, param, or body decorators to handle optional data.

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