What is the JavaScript module bundler ecosystem?

Module bundlers combine JavaScript modules (and their dependencies) into optimized files for production. Webpack — the most widely used; highly configurable; transforms assets (CSS, images, fonts) via loaders; optimizes via plugins; supports code splitting. Complex configuration but very powerful. Vite — modern build tool using native ES modules for development (no bundling during dev, instant HMR) and Rollup for production builds. Much faster developer experience than Webpack. Default for Vue, and increasingly for React. Rollup — excellent for libraries; produces clean, efficient bundles with tree-shaking; less complexity than Webpack. esbuild — written in Go; extremely fast (100x faster than Webpack); used as a sub-dependency in Vite/Parcel. Parcel — zero-configuration bundler; great for quick projects. Turbopack — Next.js's new Rust-based bundler (successor to Webpack). Key optimizations all bundlers provide: tree-shaking (remove unused code), code splitting (lazy-load chunks), minification, and module federation.