Express.js (also known as Express) is a popular open-source web application framework for Node.js. It provides a set of features and tools for building web applications and APIs, making it an ideal choice for developing scalable and robust web services. Express.js emphasizes simplicity, modularity, and flexibility, enabling developers to customize and extend its core functionality as needed. It includes built-in support for features such as routing, middleware, templating engines, and static file serving, as well as for integrating with external systems such as databases and authentication providers. Additionally, Express.js has a large and active community of developers, which has contributed a wide range of plugins and extensions to the framework. With its rich feature set, flexibility, and community support, Express.js has become one of the most popular frameworks for building web applications and APIs in Node.js.