**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Error Handling and Validation **Topic:** Understanding validation decorators and validation schemas In this topic, we will delve into the world of validation in NestJS, exploring the various validation decorators and schemas that help ensure the integrity and accuracy of our application's data. ### Validation Decorators Validation decorators are used to validate the input data of a controller method or a service method. They are applied to the properties of a class or an object to check if the values meet certain criteria. #### @IsString() The `@IsString()` decorator checks if a value is a string. ```typescript import { IsString } from 'class-validator'; export class CreateUserDto { @IsString() name: string; } ``` #### @IsNumber() The `@IsNumber()` decorator checks if a value is a number. ```typescript import { IsNumber } from 'class-validator'; export class CreateUserDto { @IsNumber() age: number; } ``` #### @IsBoolean() The `@IsBoolean()` decorator checks if a value is a boolean. ```typescript import { IsBoolean } from 'class-validator'; export class CreateUserDto { @IsBoolean() isAdmin: boolean; } ``` #### @IsArray() The `@IsArray()` decorator checks if a value is an array. ```typescript import { IsArray } from 'class-validator'; export class CreateUserDto { @IsArray() skills: string[]; } ``` #### @IsEnum() The `@IsEnum()` decorator checks if a value is an enum. ```typescript import { IsEnum } from 'class-validator'; enum Role { ADMIN, USER, } export class CreateUserDto { @IsEnum(Role) role: Role; } ``` #### @IsIn() The `@IsIn()` decorator checks if a value is in a list of values. ```typescript import { IsIn } from 'class-validator'; export class CreateUserDto { @IsIn(['admin', 'user']) role: string; } ``` #### @IsNotIn() The `@IsNotIn()` decorator checks if a value is not in a list of values. ```typescript import { IsNotIn } from 'class-validator'; export class CreateUserDto { @IsNotIn(['admin', 'user']) role: string; } ``` #### @IsEmail() The `@IsEmail()` decorator checks if a value is an email. ```typescript import { IsEmail } from 'class-validator'; export class CreateUserDto { @IsEmail() email: string; } ``` #### @IsUrl() The `@IsUrl()` decorator checks if a value is a URL. ```typescript import { IsUrl } from 'class-validator'; export class CreateUserDto { @IsUrl() website: string; } ``` #### @IsUUID() The `@IsUUID()` decorator checks if a value is a UUID. ```typescript import { IsUUID } from 'class-validator'; export class CreateUserDto { @IsUUID() id: string; } ``` #### @IsObject() The `@IsObject()` decorator checks if a value is an object. ```typescript import { IsObject } from 'class-validator'; export class CreateUserDto { @IsObject() address: { street: string; city: string; state: string; zip: string; }; } ``` ### Validation Schemas Validation schemas are used to define a set of validation rules for a class or an object. #### Create a validation schema ```typescript import { validate } from 'class-validator'; export class CreateUserDto { name: string; age: number; isAdmin: boolean; } const createUserDto = new CreateUserDto(); createUserDto.name = 'John Doe'; createUserDto.age = 30; createUserDto.isAdmin = true; validate(createUserDto).then((errors) => { if (errors.length > 0) { console.log('Validation failed:', errors); } else { console.log('Validation successful'); } }); ``` #### Use a validation schema in a controller ```typescript import { Controller, Post, Body } from '@nestjs/common'; import { CreateUserDto } from './create-user.dto'; @Controller('users') export class UsersController { @Post() createUser(@Body() createUserDto: CreateUserDto) { // Validate the user data return createUserDto; } } ``` ### Conclusion In this topic, we have covered the various validation decorators and schemas available in NestJS. We have seen how to use these decorators and schemas to validate the input data of a controller method or a service method. We have also seen how to use a validation schema in a controller to validate the user data. ### Exercise Create a validation schema for a user entity with the following properties: * name: string * age: number * isAdmin: boolean * address: { street: string city: string state: string zip: string } Use the validation schema to validate a user object. ### Solution ```typescript import { validate } from 'class-validator'; export class User { name: string; age: number; isAdmin: boolean; address: { street: string; city: string; state: string; zip: string; }; } const user = new User(); user.name = 'John Doe'; user.age = 30; user.isAdmin = true; user.address = { street: '123 Main St', city: 'Anytown', state: 'CA', zip: '12345', }; validate(user).then((errors) => { if (errors.length > 0) { console.log('Validation failed:', errors); } else { console.log('Validation successful'); } }); ``` ### External Resources * [Class Validator](https://github.com/typestack/class-validator) * [NestJS Documentation](https://docs.nestjs.com/) ### Leave a comment or ask for help If you have any questions or need help with the topic, please leave a comment below.