Framework

A framework is a structured foundation for building software that supplies architecture, conventions, and common functionality so a developer writes only what is specific to the application. The distinction from a library is control flow: you call a library, whereas a framework calls your code at the points it defines, an arrangement often described as inversion of control. That trade gives up some freedom in exchange for routing, data fetching, build configuration, and testing that already work together. In the JavaScript ecosystem, React is a UI library while Next.js is the framework wrapped around it, adding routing, server rendering, and build tooling; Fastify and NestJS play the same role on the server, and Django and Rails do so in Python and Ruby. AI development has its own layer, with LangChain and LlamaIndex structuring retrieval and agent workflows. The costs are lock-in and upgrade work, since major versions can change core assumptions and a framework's conventions shape a codebase permanently. Choosing one on popularity alone is the usual error; the practical questions are whether its defaults match the product and whether the team can maintain it for years.