ValidationPipe in depth in NestJS - Mini Project: Build & Apply

ValidationPipe in depth

📖 Scenario: You are building a simple NestJS API to accept user registration data. You want to ensure the data is valid before processing it.

🎯 Goal: Build a NestJS controller that uses ValidationPipe to validate incoming user data with DTOs and class-validator decorators.

📋 What You'll Learn

Create a User DTO class with validation decorators

Set up a ValidationPipe instance with whitelist enabled

Use ValidationPipe in the controller to validate incoming data

Return the validated user data from the controller method

💡 Why This Matters

🌍 Real World

Validating user input in APIs is essential to prevent bad data and security issues. ValidationPipe helps automate this in NestJS.

💼 Career

Backend developers often use validation pipes to ensure data integrity and improve API reliability.

1

Create User DTO with validation decorators

Create a class called CreateUserDto with three properties: username (string, minimum length 3), email (string, must be an email), and age (number, minimum 18). Use class-validator decorators @IsString(), @MinLength(3), @IsEmail(), and @Min(18) as appropriate.

NestJS

import { IsString, MinLength, IsEmail, Min } from 'class-validator';

// Your code here

Hint

Use class-validator decorators on each property to enforce the rules.

2

Configure ValidationPipe with whitelist

Create a constant called validationPipe that is a new instance of ValidationPipe with the option whitelist: true.

NestJS

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

import { IsString, MinLength, IsEmail, Min } from 'class-validator';

export class CreateUserDto {
  @IsString()
  @MinLength(3)
  username: string;

  @IsEmail()
  email: string;

  @Min(18)
  age: number;
}

// Your code here

Hint

Use new ValidationPipe({ whitelist: true }) to create the pipe.

3

Use ValidationPipe in controller method

Create a NestJS controller class called UserController with a method createUser that accepts a body parameter of type CreateUserDto. Use the @Body() decorator with the validationPipe to validate the input. The method should return the validated body.

NestJS

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

import { CreateUserDto, validationPipe } from './dto';

// Your code here

Hint

Use @Body(validationPipe) to apply validation on the input DTO.

4

Complete module setup with controller

Create a NestJS module class called UserModule that imports Module from '@nestjs/common' and declares the UserController in its controllers array.

NestJS

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

// Your code here

Hint

Use @Module({ controllers: [UserController] }) to declare the controller.

Practice

1. What is the main purpose of ValidationPipe in a NestJS application?

easy

A. To check and clean incoming data before it is used

B. To handle database connections automatically

C. To manage user authentication and sessions

D. To serve static files like images and CSS

Solution

1. Step 1: Understand ValidationPipe role

   ValidationPipe is designed to validate and sanitize incoming request data to ensure it meets expected rules.

2. Step 2: Compare with other options

   Options B, C, and D relate to other parts of NestJS or web apps, not data validation.

3. Final Answer:

   To check and clean incoming data before it is used -> Option A

4. Quick Check:

   ValidationPipe purpose = data validation [OK]

Hint: ValidationPipe = data check and clean before use [OK]

Common Mistakes:

• Confusing ValidationPipe with authentication or database handling
• Thinking ValidationPipe serves static files
• Assuming ValidationPipe manages sessions

2. Which of the following is the correct way to enable automatic transformation of input data types using ValidationPipe in NestJS?

easy

A. new ValidationPipe({ forbidNonWhitelisted: true })

B. new ValidationPipe({ whitelist: true })

C. new ValidationPipe({ transform: true })

D. new ValidationPipe({ skipMissingProperties: true })

Solution

1. Step 1: Identify transform option

   The transform: true option enables automatic conversion of input data types to expected types.

2. Step 2: Understand other options

   whitelist removes unwanted properties, forbidNonWhitelisted throws error on extra props, skipMissingProperties skips validation for missing props.

3. Final Answer:

   new ValidationPipe({ transform: true }) -> Option C

4. Quick Check:

   transform option enables type conversion [OK]

Hint: Use transform: true to convert input types automatically [OK]

Common Mistakes:

• Confusing whitelist with transform
• Using forbidNonWhitelisted to transform data
• Thinking skipMissingProperties enables transformation

3. Given this DTO and ValidationPipe setup, what will be the output when sending { "age": "25" } in the request body?

class UserDto {
  @IsInt()
  age: number;
}

// In controller
@Post()
create(@Body(new ValidationPipe({ transform: true })) user: UserDto) {
  return typeof user.age;
}

medium

A. "string"

B. "undefined"

C. Validation error thrown

D. "number"

Solution

1. Step 1: Check ValidationPipe transform option

   Transform is true, so input string "25" will be converted to number 25.

2. Step 2: Determine returned type

   Since age is transformed to number, typeof user.age returns "number".

3. Final Answer:

   "number" -> Option D

4. Quick Check:

   transform converts string to number, so typeof = "number" [OK]

Hint: transform: true converts strings to numbers automatically [OK]

Common Mistakes:

• Assuming input stays string without transform
• Expecting validation error for string number
• Thinking age is undefined without explicit conversion

4. Identify the error in this ValidationPipe usage that causes unexpected behavior:

@Post()
create(@Body(new ValidationPipe({ whitelist: true })) user: UserDto) {
  return user;
}

// UserDto has properties: name (string), age (number)

Request body: { "name": "Alice", "age": 30, "extra": "value" }

medium

A. The ValidationPipe should have forbidNonWhitelisted to throw error on extra props

B. The whitelist option removes the extra property silently

C. The ValidationPipe is missing transform: true to convert types

D. The extra property causes a validation error

Solution

1. Step 1: Understand whitelist behavior

   Whitelist removes properties not in DTO silently without error.

2. Step 2: Check how to throw error on extra props

   To throw error on extra properties, forbidNonWhitelisted: true must be set.

3. Final Answer:

   The ValidationPipe should have forbidNonWhitelisted to throw error on extra props -> Option A

4. Quick Check:

   forbidNonWhitelisted throws error on extra props [OK]

Hint: Use forbidNonWhitelisted to reject extra properties with error [OK]

Common Mistakes:

• Thinking whitelist throws error by default
• Ignoring need for forbidNonWhitelisted option
• Confusing whitelist with transform option

5. You want to ensure that your NestJS API only accepts properties defined in your DTO, automatically converts input types, and throws an error if extra properties are sent. Which ValidationPipe configuration achieves this?

hard

A. new ValidationPipe({ whitelist: true, transform: true })

B. new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true })

C. new ValidationPipe({ transform: true, skipMissingProperties: true })

D. new ValidationPipe({ whitelist: true, forbidNonWhitelisted: true })

Solution

1. Step 1: Identify needed options

   Whitelist removes extra props, transform converts types, forbidNonWhitelisted throws error on extra props.

2. Step 2: Match options to requirements

   Only new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true }) includes all three: whitelist, transform, and forbidNonWhitelisted.

3. Final Answer:

   new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true }) -> Option B

4. Quick Check:

   Whitelist + transform + forbidNonWhitelisted = strict validation [OK]

Hint: Combine whitelist, transform, forbidNonWhitelisted for strict validation [OK]

Common Mistakes:

• Omitting forbidNonWhitelisted to throw errors
• Forgetting transform for type conversion
• Using skipMissingProperties instead of forbidNonWhitelisted