class-validator Cheatsheet: Useful Decorators and NestJS Validation Patterns (2025)

August 4, 2025

This content originally appeared on DEV Community and was authored by Subhash Adhikari

This is your complete reference for all major class-validator decorators, their usage, value types, and integration patterns. Bookmark this if you’re working with DTOs in NestJS or doing input validation in any TypeScript project.

Install First

npm install class-validator class-transformer

Basic Usage Pattern

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

class LoginDto {
  @IsEmail()
  email: string;

  @MinLength(6)
  password: string;
}

const login = new LoginDto();
login.email = 'invalid';
login.password = '123';

const errors = await validate(login);

Core Decorators Reference

Decorator	Purpose	Accepted Type(s)
`@IsString()`	Must be a string	`string`
`@IsBoolean()`	Must be boolean	`boolean`
`@IsInt()`	Must be an integer	`number`
`@IsNumber()`	Any number (int or float)	`number`
`@IsEmail()`	Valid email address	`string`
`@IsDate()`	Must be a Date instance	`Date`
`@IsOptional()`	Skip validation if undefined/null	`any`
`@IsNotEmpty()`	Not empty (works with string, array)	`string`, `array`
`@IsDefined()`	Value must be present (not undefined)	`any`

Number Validations

Decorator	Description
`@Min(value)`	Minimum number allowed
`@Max(value)`	Maximum number allowed
`@IsPositive()`	Must be positive
`@IsNegative()`	Must be negative

String Validations

Decorator	Description
`@MinLength(n)`	Minimum string length
`@MaxLength(n)`	Maximum string length
`@Matches(regex)`	Must match regex pattern
`@IsAlpha()`	Only letters
`@IsAlphanumeric()`	Letters and numbers only
`@IsPhoneNumber()`	Must be a valid phone number
`@IsUrl()`	Must be a valid URL

Array Validations

Decorator	Description
`@IsArray()`	Must be an array
`@ArrayMinSize(n)`	Minimum length of array
`@ArrayMaxSize(n)`	Maximum length of array
`@ArrayNotEmpty()`	Must not be empty
`@IsInt({ each: true })`	Validates each element (works with many)

Enum & Object Validations

enum Role { USER = 'user', ADMIN = 'admin' }

@IsEnum(Role)
role: Role;

@ValidateNested()
@Type(() => Profile)
profile: Profile;

@ValidateNested() is required for nested objects
Must be combined with @Type(() => ClassName) from class-transformer

Conditional Validation

@ValidateIf(o => o.isAdult)
@IsString()
licenseNumber: string;

Use @ValidateIf() to apply a rule conditionally based on other values.

Custom Validators

@ValidatorConstraint({ name: 'IsStrongPassword', async: false })
class IsStrongPassword implements ValidatorConstraintInterface {
  validate(value: string) {
    return typeof value === 'string' && value.length >= 8 && /[A-Z]/.test(value);
  }

  defaultMessage() {
    return 'Password must be at least 8 chars and include an uppercase letter.';
  }
}

Use it like this:

@Validate(IsStrongPassword)
password: string;

NestJS Global Integration

// main.ts
app.useGlobalPipes(new ValidationPipe({
  whitelist: true,
  forbidNonWhitelisted: true,
  transform: true,
  transformOptions: { enableImplicitConversion: true }
}));