What is the difference between a transformation pipe and a validation pipe in NestJS?

Q: What is the difference between a transformation pipe and a validation pipe in NestJS?

// Transformation — handler receives number, not string @Get(':id') findOne(@Param('id', ParseIntPipe) id: number) {} // Validation — handler receives validated DTO or gets 400 @Post() create(@Body(new ValidationPipe()) dto: CreateUserDto) {} // Both — validates AND transforms in one pipe @Injectable() export class ParsePositiveIntPipe implements PipeTransform { transform(value: string): number { const n = parseInt(value, 10); if (isNaN(n) || n <= 0) throw new BadRequestException('Must be a positive integer'); return n; // returns number, validates positivity } } Transformation pipe — changes the type or shape of the value; handler receives the new type. Validation pipe — asserts rules about the value; throws BadRequestException on failure. A single pipe can both validate and transform — validate first, then return the transformed value. Built-in transformation pipes: ParseIntPipe, ParseFloatPipe, ParseBoolPipe, ParseUUIDPipe. Built-in validation pipe: ValidationPipe using class-validator decorators.

Difficulty: 5/10

pipes, validation, transformation

A transformation pipe converts the input to a different type and returns the new value — the handler receives the transformed type, not the raw string. A validation pipe checks the input against rules and either returns it unchanged or throws BadRequestException. A pipe can do both simultaneously.

Transformation vs validation pipe examples

Key differences:

1
Transformation pipe — changes the type or shape of the value; handler receives the new type.
2
Validation pipe — asserts rules about the value; throws BadRequestException on failure.
3
A single pipe can both validate and transform — validate first, then return the transformed value.
4
Built-in transformation pipes: ParseIntPipe, ParseFloatPipe, ParseBoolPipe, ParseUUIDPipe.
5
Built-in validation pipe: ValidationPipe using class-validator decorators.

Scenario Questions

0-2 years experience

2-5 years experience

Follow-up Questions