Validation with Joi in NestJS - Mini Project: Build & Apply

Validation with Joi in NestJS

📖 Scenario: You are building a simple NestJS API to register users. You want to make sure the data sent by clients is correct before saving it.

🎯 Goal: Create a Joi validation schema to check user data, then apply it in a NestJS DTO to validate incoming requests.

📋 What You'll Learn

Create a Joi schema for user data with exact fields

Add a configuration variable for minimum password length

Use the Joi schema in a NestJS DTO class

Apply the validation pipe in the controller method

💡 Why This Matters

🌍 Real World

Validating user input is essential in APIs to prevent bad data and security issues. Joi is a popular library for this in Node.js and NestJS.

💼 Career

Backend developers often use validation libraries like Joi to ensure data integrity and improve API reliability.

1

Create Joi schema for user data

Create a constant called userSchema using Joi.object() with these exact fields: username as a required string, email as a required string with email format, and password as a required string.

NestJS

# Your code here

Hint

Use Joi.object() to define the schema and Joi.string().required() for each field. For email, add .email().

2

Add minimum password length configuration

Create a constant called MIN_PASSWORD_LENGTH and set it to 8. Then update the password field in userSchema to require a minimum length using .min(MIN_PASSWORD_LENGTH).

NestJS

import * as Joi from 'joi';

export const userSchema = Joi.object({
  username: Joi.string().required(),
  email: Joi.string().email().required(),
  password: Joi.string().required(),
});

# Your code here

Hint

Define MIN_PASSWORD_LENGTH as 8, then use .min(MIN_PASSWORD_LENGTH) on the password field.

3

Use Joi schema in NestJS DTO

Create a class called CreateUserDto. Add a static property called schema and assign it the userSchema constant.

NestJS

import * as Joi from 'joi';

export const MIN_PASSWORD_LENGTH = 8;

export const userSchema = Joi.object({
  username: Joi.string().required(),
  email: Joi.string().email().required(),
  password: Joi.string().min(MIN_PASSWORD_LENGTH).required(),
});

# Your code here

Hint

Create a class and add a static property schema that points to userSchema.

4

Apply validation pipe in controller

In a NestJS controller method called createUser, use the @UsePipes(new JoiValidationPipe(CreateUserDto.schema)) decorator above the method. The method should accept a parameter createUserDto of type CreateUserDto.

NestJS

import * as Joi from 'joi';
import { Controller, Post, Body, UsePipes } from '@nestjs/common';

export const MIN_PASSWORD_LENGTH = 8;

export const userSchema = Joi.object({
  username: Joi.string().required(),
  email: Joi.string().email().required(),
  password: Joi.string().min(MIN_PASSWORD_LENGTH).required(),
});

export class CreateUserDto {
  static schema = userSchema;
}

@Controller('users')
export class UsersController {
  
  # Your code here
}

Hint

Use @UsePipes(new JoiValidationPipe(CreateUserDto.schema)) above the createUser method and accept createUserDto as a parameter with @Body().

Practice

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

easy

A. To style the user interface

B. To manage database connections

C. To handle HTTP routing

D. To validate and enforce rules on incoming data

Solution

1. Step 1: Understand Joi's role

   Joi is a library used to define schemas that describe how data should look and to check if data matches those rules.

2. Step 2: Identify the correct purpose in NestJS

   In NestJS, Joi is used to validate incoming data to ensure it meets expected formats and constraints before processing.

3. Final Answer:

   To validate and enforce rules on incoming data -> Option D

4. Quick Check:

   Joi = Data validation [OK]

Hint: Joi is for checking data shapes and rules [OK]

Common Mistakes:

• Confusing Joi with database or routing tools
• Thinking Joi styles UI elements
• Assuming Joi manages server configuration

2. Which of the following is the correct way to define a Joi schema for a required string field named username?

easy

A. Joi.string().required()

B. Joi.number().required()

C. Joi.boolean().optional()

D. Joi.array().required()

Solution

1. Step 1: Identify the data type for username

   A username is typically a string, so the schema should start with Joi.string().

2. Step 2: Make the field required

   Adding .required() ensures the field must be present and not empty.

3. Final Answer:

   Joi.string().required() -> Option A

4. Quick Check:

   String + required = Joi.string().required() [OK]

Hint: Use Joi.string().required() for mandatory text fields [OK]

Common Mistakes:

• Using wrong Joi types like number or boolean for strings
• Forgetting to add .required() for mandatory fields
• Confusing optional() with required()

3. Given this Joi schema:

const schema = Joi.object({ age: Joi.number().min(18).required() });
const result = schema.validate({ age: 16 });

What will result.error contain?

medium

A. No error, validation passes

B. An error indicating age must be at least 18

C. An error indicating age is missing

D. An error indicating age must be a string

Solution

1. Step 1: Analyze the schema rules

   The schema requires age to be a number, at least 18, and required.

2. Step 2: Check the input data

   The input has age: 16, which is less than the minimum 18.

3. Step 3: Determine validation result

   Validation fails because 16 is below the minimum, so result.error contains a message about age being too low.

4. Final Answer:

   An error indicating age must be at least 18 -> Option B

5. Quick Check:

   Age 16 < 18 = validation error [OK]

Hint: Check min() rules against input values [OK]

Common Mistakes:

• Assuming 16 passes min(18) validation
• Confusing missing field error with min value error
• Expecting string type error instead of number

4. Identify the error in this Joi schema definition:

const schema = Joi.object({
  email: Joi.string().email,
  password: Joi.string().min(8).required()
});

medium

A. email should be Joi.email() instead of Joi.string().email

B. password should not be required

C. Missing parentheses after email() method

D. min(8) is invalid for strings

Solution

1. Step 1: Review the email field syntax

   The email method in Joi is a function and must be called with parentheses: email().

2. Step 2: Identify the missing parentheses

   The code uses Joi.string().email without parentheses, which is incorrect and will cause an error.

3. Final Answer:

   Missing parentheses after email() method -> Option C

4. Quick Check:

   Joi methods require () to execute [OK]

Hint: Always add () after Joi methods like email() [OK]

Common Mistakes:

• Omitting parentheses on Joi method calls
• Misunderstanding required() necessity
• Thinking min() is invalid for strings

5. You want to validate a user object with name (required string), age (optional number, minimum 0), and email (required valid email). Which Joi schema correctly applies these rules?

hard

A. Joi.object({ name: Joi.string().required(), age: Joi.number().min(0), email: Joi.string().email().required() })

B. Joi.object({ name: Joi.string(), age: Joi.number().min(0).required(), email: Joi.string().email() })

C. Joi.object({ name: Joi.string().required(), age: Joi.number().min(1).optional(), email: Joi.string().email().required() })

D. Joi.object({ name: Joi.string().required(), age: Joi.number().min(0).required(), email: Joi.string().email().required() })

Solution

1. Step 1: Check each field's requirements

   Name must be a required string, age is optional number with min 0, email is required valid email string.

2. Step 2: Match schema to requirements

   Joi.object({ name: Joi.string().required(), age: Joi.number().min(0), email: Joi.string().email().required() }) matches all: name is required string, age is number with min 0 (optional by default), email is required email string.

3. Step 3: Verify other options

   Joi.object({ name: Joi.string(), age: Joi.number().min(0).required(), email: Joi.string().email() }) makes age required and email optional, Joi.object({ name: Joi.string().required(), age: Joi.number().min(1).optional(), email: Joi.string().email().required() }) uses min(1) for age (wrong min), Joi.object({ name: Joi.string().required(), age: Joi.number().min(0).required(), email: Joi.string().email().required() }) makes age required (wrong).

4. Final Answer:

   Joi.object({ name: Joi.string().required(), age: Joi.number().min(0), email: Joi.string().email().required() }) -> Option A

5. Quick Check:

   Required fields + optional age min 0 = Joi.object({ name: Joi.string().required(), age: Joi.number().min(0), email: Joi.string().email().required() }) [OK]

Hint: Required fields use .required(), optional omit it [OK]

Common Mistakes:

• Making optional fields required by mistake
• Using wrong min values for age
• Forgetting to call email() with parentheses