Default value pipe in NestJS - Mini Project: Build & Apply

Using Default Value Pipe in NestJS

📖 Scenario: You are building a simple NestJS controller to handle user queries. Sometimes users may not provide a query parameter, so you want to set a default value automatically.

🎯 Goal: Create a NestJS controller with a GET endpoint that uses the DefaultValuePipe to provide a default value for a query parameter called page.

📋 What You'll Learn

Create a controller class named AppController.

Add a GET route handler method named getItems.

Use the @Query decorator with DefaultValuePipe to set a default value of 1 for the page query parameter.

Return the page value as a number from the handler.

💡 Why This Matters

🌍 Real World

Setting default values for query parameters is common in APIs to handle missing inputs gracefully.

💼 Career

Understanding pipes like DefaultValuePipe is essential for building robust and user-friendly APIs in NestJS.

1

Create the controller class and import necessary decorators

Create a class called AppController and import Controller and Get from @nestjs/common.

NestJS

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

// Your code here

Hint

Use export class AppController and decorate it with @Controller().

2

Add the GET route handler method

Inside AppController, add a method named getItems decorated with @Get() that returns 0 for now.

NestJS

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

@Controller()
export class AppController {
  // Your code here
}

Hint

Define getItems() method and decorate it with @Get().

3

Import and use DefaultValuePipe for the query parameter

Import Query and DefaultValuePipe from @nestjs/common. Modify getItems to accept a page parameter from query using @Query('page', new DefaultValuePipe('1')).

NestJS

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

@Controller()
export class AppController {
  @Get()
  getItems(
    // Your code here
  ) {
    return 0;
  }
}

Hint

Use @Query('page', new DefaultValuePipe('1')) to set default page value.

4

Convert the page parameter to a number before returning

Modify the getItems method to convert the page parameter from string to number using Number(page) and return it.

NestJS

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

@Controller()
export class AppController {
  @Get()
  getItems(
    @Query('page', new DefaultValuePipe('1')) page: string
  ) {
    // Your code here
  }
}

Hint

Use Number(page) to convert the string to a number before returning.

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