Gatsby.js is a framework to write web applications in an architectural style known as JAMstack. Part of the application is generated at build time and deployed as a static site, while the remainder of the functionality is implemented as a progressive web application (PWA) running in the browser. Such applications work without code running on the server side. Usually, though, the PWA makes calls to third-party APIs and SaaS solutions for content management, for example. In the case of Gatsby.js, all client and build time code is written using React. The framework includes some optimizations to make the web application feel fast. It provides code and data splitting out of the box to minimize load times and speeds up performance when navigating the application by prefetching resources. APIs are called via GraphQL and several plugins simplify integration with existing services.