Why pipes transform and validate input in NestJS - See It in Action

Why pipes transform and validate input

📖 Scenario: You are building a simple NestJS API that accepts user data. You want to make sure the data is correct and in the right format before your app uses it.

🎯 Goal: Create a NestJS pipe that transforms and validates user input. This pipe will check if the input is a number and convert it from a string to a number. If the input is not a valid number, it will throw an error.

📋 What You'll Learn

Create a pipe class named ParseIntPipe

Add a transform method that converts input to a number

Validate that the input is a number after conversion

Throw an error if the input is not a valid number

💡 Why This Matters

🌍 Real World

APIs often receive data as strings from users or other systems. Pipes help convert and check this data before your app uses it.

💼 Career

Understanding pipes is essential for building robust NestJS applications that handle input safely and correctly.

1

Create the pipe class

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

NestJS

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

// Your code here

Need a hint?

Use implements PipeTransform to create a pipe class.

2

Add transformation logic

Inside the transform method, convert the input value to a number using parseInt(value, 10) and store it in a variable called val.

NestJS

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

export class ParseIntPipe implements PipeTransform {
  transform(value: any) {
    // Convert value to number
    // Your code here
  }
}

Need a hint?

Use parseInt with base 10 to convert the string to a number.

3

Add validation for number

After converting, check if val is NaN using isNaN(val). If it is, throw a BadRequestException from @nestjs/common with the message 'Validation failed: Not a number'.

NestJS

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

export class ParseIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseInt(value, 10);
    // Validate val is a number
    // Your code here
  }
}

Need a hint?

Use BadRequestException to signal invalid input.

4

Return the transformed value

At the end of the transform method, return the variable val so the transformed number is passed on.

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('Validation failed: Not a number');
    }
    // Return the number
    // Your code here
  }
}

Need a hint?

Return the transformed value so the next part of the app receives a number.