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

🧠 Common Mistakes to Avoid

❌ Forgetting @Type(): Nested validation won’t work without it
❌ Validating plain objects: Use plainToInstance() from class-transformer
❌ Wrong order of decorators: Execution is bottom-to-top, so arrange carefully

📦 Pro Tips

Combine @IsOptional() with other validators to skip when empty
Use { each: true } to apply decorators to every array element
Custom validators = reusable logic for business rules
Enable stopAtFirstError in ValidationPipe to fail fast

🔗 Resources

✅ Use this cheatsheet in every TypeScript API project using DTOs. Fast, reliable, and easy to maintain.

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

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Subhash Adhikari | Sciencx (2025-08-04T21:03:29+00:00) class-validator Cheatsheet: Useful Decorators and NestJS Validation Patterns (2025). Retrieved from https://www.scien.cx/2025/08/04/class-validator-cheatsheet-useful-decorators-and-nestjs-validation-patterns-2025/

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