PipeTransform is the generic interface (PipeTransform<T, R>) that any NestJS pipe must implement. It defines the transform(value: T, metadata: ArgumentMetadata): R method, where T is the input type and R is the return type.
The transform() method receives the current method argument before the route handler executes and returns the transformed or validated value. A validation pipe returns the value unchanged; a transformation pipe returns a modified value.
Any operation (validation or transformation) takes place before the route handler is invoked, and if an exception is thrown, the handler never executes.