Nest JS Class Validator & Class Transformers

They are mostly used to validate and transform the incoming request body.

• Class Validator : It validates incoming request against defined rules.

export class sendMessageDTO{

    @IsString();
    @IsNotEmpty()
    message:string;

    @IsMongoId();
    user:Mongoose.ObjectId;

}

Some useful Class Validator Decorators

• @IsString()
• @IsNotEmpty()
• @IsMongoId()
• @IsNumber()
• @IsEmail()
• @IsBoolean()
• @IsDate()
• @IsArray()
• @IsEnum()
• @IsOptional()
• @isin()
• @IsInstance()
• @IsObject()
• @IsUrl()
• @IsUUID()
• @IsDateString()
• @IsNumberString()
• @IsAlpha()
• @ValidatedNested()
• @IsAlphanumeric()

Class Transformer : It converts plain JS Object into class instance.

    // This will convert req.body into sendMessageDTO instance.
    // And then we can use class validator to validate it.
    const message = plainToInstance(sendMessageDTO,req.body);
    // If we do this 
    // Then transformations happen automatically in NEST JS from object to class Instance.
    app.useGlobalPipe(new ValidationPipe({transform:true}));

• They are used when we have to validate Nested fields.
  

  class UserDTO{
      @IsString()
      @IsNotEmpty()
      name:string;
  }
  
  class SendMessageDTO{
      @IsString();
      @IsNotEmpty()
      message:string;
  
      @ValidateNested()
      user:UserDTO;
  }
  
  // BTS 
  // we got the request Body 
  // const data = {
  //     message:"Hello",
  //     user:{
  //         name:"Jayant"
  //     }
  //};
  
  // Then we convert it into the classInstance
  // const sendMessageDto = plainToInstance(SendMessageDTO,data);
  // Now our class instance, sendMessageDto will have user as simple object.
  // But we want user to be an instance of UserDTO.
  // For that we have to use class-transformer.
  // We have to use @Type() decorator.
  // @Type(() => UserDTO)
  // user:UserDTO;
  
  // Correct APPROACH :
  class SendMessageDTO{
      @IsString();
      @IsNotEmpty()
      message:string;
  
      @ValidateNested()
      @Type(()=>UserDTO)  // Imported from class-transformer
      user:UserDTO;
  }
  

• We can also do Data Transformation inside the DTO
  

  class UserDTO{
      @Transform(({value})=>value.toLowerCase())
      name:string;
  }
  

Bonus

• All this is possible due to reflect-metadata package, that allows us to attach metadata to class properties and methods. It works BTS to attach metadata to class properties and methods as decorators weren't available in JS.

• In Controller when we do @Body sendMessage:SendMessageDTO we already got the transformed & validated, if we are using ValidationPipe.