Custom pipes in NestJS

Introduction

Custom pipes let you change or check data before your code uses it. They help keep your app clean and safe.

You want to check if user input is correct before saving it.

You need to change data format, like turning a string into a number.

You want to block bad data early to avoid errors later.

You want to reuse the same data check in many places.

You want to keep your main code simple by moving data checks out.

Syntax

NestJS

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

@Injectable()
export class CustomPipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    // change or check value here
    return value; // return changed or original value
  }
}

The transform method runs automatically on data.

Throw BadRequestException to stop bad data.

Examples

This pipe changes a string to a number and stops if it is not a number.

NestJS

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

export class ParseIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseInt(value, 10);
    if (isNaN(val)) {
      throw new BadRequestException('Value must be a number');
    }
    return val;
  }
}

This pipe removes spaces from the start and end of a string.

NestJS

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

export class TrimPipe implements PipeTransform {
  transform(value: any) {
    if (typeof value === 'string') {
      return value.trim();
    }
    return value;
  }
}

Sample Program

This example shows a controller with a custom pipe that changes the query parameter id from a string to a number. If id is not a number, it stops the request with an error.

NestJS

import { Controller, Get, Query, UsePipes, PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
class ParseIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseInt(value, 10);
    if (isNaN(val)) {
      throw new BadRequestException('Value must be a number');
    }
    return val;
  }
}

@Controller('items')
export class ItemsController {
  @Get()
  @UsePipes(new ParseIntPipe())
  getItem(@Query('id') id: number) {
    return `Item id is ${id}`;
  }
}

Important Notes

Always add @Injectable() to your custom pipe class.

Use pipes to keep your controller code clean and focused on business logic.

You can apply pipes globally, to controllers, or to specific routes.

Summary

Custom pipes let you check or change data before your code uses it.

They help catch errors early and keep your app safe.

You write a class with a transform method to make a custom pipe.

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