Express

Express (sometimes referred to as ExpressJS) is an open-source backend framework for Node.js. It is one of the earliest frameworks for Node.js and has since it release in 2010 become one of the most popular ways of building application backends for the Node.js runtime.

Though Express' primary purpose is to build REST APIs, it is highly flexible and easy to pair with many third-party libraries in the Node.js ecosystem. This, for example, makes Express a viable option to building GraphQL APIs, serving server-side rendered templates or handling role-based authentication.

Express' biggest strength is its flexibility and maturity, which is also a reason that many Node.js backend frameworks are built on top of Express - this includes popular frameworks such as NestJS, Sails and Kraken.