What is Hot Module Replacement (HMR)?
A development feature that swaps updated modules into a running app without a full reload, preserving application state.
Definition
Hot module replacement injects edited code into the live page over a WebSocket connection instead of triggering a browser refresh, so form inputs, component state, and scroll position survive an edit. Vite's HMR is near-instant because it serves native ES modules and invalidates only the changed file's import chain; Webpack's dev server pioneered the approach. Framework integrations add state-preserving semantics: React Fast Refresh re-renders edited components while keeping hooks state, and Vue's HMR hot-swaps single-file components. When a module can't self-accept an update, HMR falls back to a full reload rather than running stale code.