Custom pipes in NestJS - Mini Project: Build & Apply

Creating a Custom Pipe in NestJS

📖 Scenario: You are building a simple NestJS API that accepts user input for age. You want to ensure the age is a valid number and transform it before it reaches your controller.

🎯 Goal: Build a custom pipe in NestJS that validates and transforms the age parameter to a number, rejecting invalid inputs.

📋 What You'll Learn

Create a custom pipe class named ParseAgePipe

Implement the PipeTransform interface

Validate that the input is a number and throw a BadRequestException if not

Transform the input string to a number

Use the custom pipe in a controller method parameter

💡 Why This Matters

🌍 Real World

Custom pipes in NestJS help validate and transform incoming data before it reaches your business logic, improving code cleanliness and error handling.

💼 Career

Understanding custom pipes is essential for backend developers working with NestJS to build robust APIs that handle user input safely and correctly.

1

Create the Custom Pipe Class

Create a class called ParseAgePipe that implements the PipeTransform interface from @nestjs/common.

NestJS

import { PipeTransform } from '@nestjs/common';

// Your code here

Hint

Start by importing PipeTransform and create a class with a transform method.

2

Add Validation Logic

Inside the transform method of ParseAgePipe, add code to check if value is a number string. If not, throw a BadRequestException from @nestjs/common with the message 'Invalid age'.

NestJS

import { PipeTransform, BadRequestException } from '@nestjs/common';

export class ParseAgePipe implements PipeTransform {
  transform(value: any) {
    // Add validation for number string
  }
}

Hint

Use Number(value) to convert and isNaN() to check validity.

3

Use the Pipe in a Controller Method

In a controller class, create a method called createUser that takes a parameter age decorated with @Body('age', new ParseAgePipe()). The method should return the age value.

NestJS

import { Controller, Post, Body } from '@nestjs/common';
import { ParseAgePipe } from './parse-age.pipe';

@Controller('users')
export class UsersController {
  // Add createUser method here
}

Hint

Use @Body('age', new ParseAgePipe()) to apply the pipe to the age parameter.

4

Complete the Pipe with Export and Import

Ensure the ParseAgePipe class is exported and imported correctly in the controller file. Confirm the controller uses the pipe in the createUser method parameter.

NestJS

import { PipeTransform, BadRequestException } from '@nestjs/common';

// Confirm export of ParseAgePipe

import { Controller, Post, Body } from '@nestjs/common';
// Confirm import of ParseAgePipe

@Controller('users')
export class UsersController {
  @Post()
  createUser(@Body('age', new ParseAgePipe()) age: number) {
    // Return age
  }
}

Hint

Make sure the pipe class is exported and imported correctly, and used in the controller method parameter.

Practice

1.

What is the main purpose of a custom pipe in NestJS?

easy

A. To transform or validate data before it reaches the route handler

B. To handle database connections automatically

C. To manage user sessions and cookies

D. To style the frontend components

Solution

1. Step 1: Understand the role of pipes in NestJS

   Pipes are used to transform or validate incoming data before it reaches the controller methods.

2. Step 2: Identify what custom pipes specifically do

   Custom pipes allow developers to create their own logic for data transformation or validation to ensure data safety and correctness.

3. Final Answer:

   To transform or validate data before it reaches the route handler -> Option A

4. Quick Check:

   Custom pipes = data validation/transformation [OK]

Hint: Pipes act on data before controller uses it [OK]

Common Mistakes:

• Confusing pipes with middleware
• Thinking pipes handle database or session logic
• Assuming pipes style frontend components

2.

Which of the following is the correct way to define a custom pipe class in NestJS?

export class MyPipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    // logic here
  }
}

easy

A. export class MyPipe implements PipeTransform { transform(value: any) { } }

B. export class MyPipe implements PipeTransform { transform(value: any, metadata: ArgumentMetadata) { } }

C. export function MyPipe(value: any) { return value; }

D. export class MyPipe { transform(value: any, metadata: ArgumentMetadata) { } }

Solution

1. Step 1: Recall the PipeTransform interface signature

   The transform method must accept two parameters: value and metadata of type ArgumentMetadata.

2. Step 2: Check which option matches the correct method signature

   export class MyPipe implements PipeTransform { transform(value: any, metadata: ArgumentMetadata) { } } correctly implements the transform method with both parameters and the class implements PipeTransform.

3. Final Answer:

   export class MyPipe implements PipeTransform { transform(value: any, metadata: ArgumentMetadata) { } } -> Option B

4. Quick Check:

   transform(value, metadata) = correct signature [OK]

Hint: transform method needs value and metadata parameters [OK]

Common Mistakes:

• Omitting the metadata parameter
• Using a function instead of a class
• Not implementing PipeTransform interface

3.

Given this custom pipe code, what will be the output if the input value is "123"?

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class ParseIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseInt(value, 10);
    if (isNaN(val)) {
      throw new BadRequestException('Validation failed');
    }
    return val;
  }
}

Input: "123"

medium

A. Returns NaN

B. Throws BadRequestException

C. Returns string "123"

D. Returns number 123

Solution

1. Step 1: Understand parseInt behavior on string "123"

   parseInt("123", 10) converts the string "123" to the number 123.

2. Step 2: Check the validation condition

   Since 123 is a valid number, isNaN(val) is false, so no exception is thrown and the number 123 is returned.

3. Final Answer:

   Returns number 123 -> Option D

4. Quick Check:

   parseInt("123") = 123 [OK]

Hint: parseInt("123") returns number 123, no error [OK]

Common Mistakes:

• Thinking it returns string instead of number
• Assuming it throws error for numeric strings
• Confusing NaN with valid number

4.

Identify the error in this custom pipe code:

import { PipeTransform, Injectable } from '@nestjs/common';

@Injectable()
export class UppercasePipe implements PipeTransform {
  transform(value: string) {
    return value.toUpperCase;
  }
}

medium

A. Missing @Injectable decorator

B. transform method missing return type

C. toUpperCase is used without parentheses

D. PipeTransform interface not implemented

Solution

1. Step 1: Check the transform method implementation

   The method returns value.toUpperCase without parentheses, which is a reference to the function, not the result.

2. Step 2: Identify correct usage of toUpperCase

   toUpperCase is a method and must be called with parentheses: value.toUpperCase()

3. Final Answer:

   toUpperCase is used without parentheses -> Option C

4. Quick Check:

   Method calls need () to execute [OK]

Hint: Remember to call string methods with () [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Assuming decorators are missing when present
• Ignoring interface implementation

5.

You want to create a custom pipe that trims whitespace and converts input to lowercase. Which of these implementations correctly achieves this?

import { PipeTransform, Injectable } from '@nestjs/common';

@Injectable()
export class NormalizePipe implements PipeTransform {
  transform(value: any) {
    // Which option is correct?
  }
}

hard

A. return value.trim().toLowerCase();

B. return value.replace(/\s/g, '').toLowerCase();

C. return value.trim.toLowerCase();

D. return value.toLowerCase.trim();

Solution

1. Step 1: Understand method chaining order

   trim() removes whitespace from start and end, then toLowerCase() converts to lowercase. The order matters to avoid errors.

2. Step 2: Check method call syntax

   Both trim() and toLowerCase() are methods and must be called with parentheses. return value.trim().toLowerCase(); correctly calls them in order: value.trim().toLowerCase()

3. Final Answer:

   return value.trim().toLowerCase(); -> Option A

4. Quick Check:

   Correct method chaining with () = return value.trim().toLowerCase(); [OK]

Hint: Call trim() before toLowerCase(), both with () [OK]

Common Mistakes:

• Calling methods without parentheses
• Wrong method chaining order
• Using properties instead of methods