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.