Hotreload

Hot reload is a development feature that allows a running application to incorporate code changes without a full restart. It aims to reduce feedback time by updating code in place and, when possible, preserving program state.

In front-end web development, hot module replacement (HMR) is a common implementation. When a source file changes, the module bundler (for example, Webpack, Vite, or Parcel) updates the corresponding module in the in-memory bundle and signals the runtime to swap the new module in. Frameworks such as React use additional logic (often called fast refresh) to preserve component state and reapply stateful updates after a change. If the update cannot be safely applied, a full page reload or a re-mount of the app may occur.

Server-side hot reload is implemented by development servers that watch the filesystem and reload code or refresh modules without restarting the entire process. Examples include Django’s autoreload and some Ruby on Rails development modes. Node.js environments sometimes simulate hot reloads using persistent processes with code injection, but many setups fall back to restarting the server.

Benefits include shorter iteration cycles and preserved state; however, hot reload is not guaranteed for all changes. Changes that alter module interfaces, initialization, or global state may require a full restart or manual state reset, and some tooling may introduce inconsistencies if updates are applied eagerly.

Hot reload is distinct from live reload, which only refreshes the user interface after changes to assets, often requiring a full page reload. In practice, many ecosystems use a combination of both to provide speedy feedback during development.