How does NestJS achieve platform agnosticism between Express and Fastify?
NestJS wraps the underlying HTTP framework behind an HttpAdapter interface. Core functionality like DI, decorators, and lifecycle is framework-independent. The adapter translates NestJS abstractions to Express or Fastify internals. You switch platforms by passing a different adapter to NestFactory.create() with no application code changes.
Switching from Express to Fastify
How platform agnosticism works:
NestJS defines an AbstractHttpAdapter interface that both Express and Fastify implement.
Controllers, guards, interceptors, and pipes work identically on both platforms.
Only platform-specific code (e.g., using @Req(), @Res() directly) breaks portability.
Fastify is significantly faster than Express but has a smaller ecosystem of middleware.