fullPage.js is a vanilla JavaScript library for creating full-screen scrolling websites (one-page sites) with vertical/horizontal snap scrolling, sections, and slides. The codebase uses a modular ES6 architecture with separate concerns for scroll handling, mouse/touch input, keyboard navigation, state management, and DOM manipulation. It exports a main class from src/js/fullpage.js with initialization via src/js/app.js. The library manages DOM structure through CSS classes (.fp-section, .fp-slide, .active, .fp-loaded) and maintains global state in src/js/common/state.js. It supports optional jQuery integration via src/js/jquery-adaptor.js and includes polyfills for IE11 compatibility. The build system uses Rollup for bundling and Gulp for asset processing, producing distribution files in dist/. The library features lazy loading for media, responsive breakpoints, and extensive callback hooks for lifecycle management.