What is the difference between FastAPI and Express?

FastAPI is a Python framework that uses type hints and Pydantic models to validate requests, serialise responses and generate OpenAPI docs automatically. Express is a minimal Node.js framework where routing and middleware are built in and validation and docs come from libraries you add.

In FastAPI, declaring a parameter or body as a Pydantic model validates and converts input automatically and returns a 422 with details on failure. In Express you add middleware such as Zod or express-validator and handle errors yourself.

FastAPI generates an OpenAPI schema and interactive Swagger UI and ReDoc pages from your code automatically. Express needs swagger-jsdoc, zod-to-openapi or a hand-written OpenAPI file.

When the team works in Python, especially for machine learning, data science or AI services, where models, pandas or PyTorch live in the same process, and when typed validation and docs out of the box are valuable.

When the team works in JavaScript or TypeScript, wants to share code and types with a React or Next.js frontend, or needs the vast npm middleware ecosystem.

FastAPI supports async def handlers on an ASGI server such as Uvicorn, and runs plain def handlers in a thread pool. Express runs on Node.js's event loop, where all I/O is asynchronous by default.

FastAPI has built-in dependencies with Depends, used for database sessions, authentication and shared logic. Express has no DI; middleware and closures pass shared objects.

Both are fast for I/O-bound APIs. FastAPI on Uvicorn is among the fastest Python frameworks; Node.js is typically faster per request, but database and network time usually dominate.

FastAPI uses response_model to filter and serialise output through Pydantic, ensuring the response matches a schema. Express sends whatever object you pass to res.json, so shaping responses is manual.

FastAPI offers security utilities for OAuth2 and API keys used through dependencies. Express uses Passport, JWT middleware or provider SDKs.