Hot Module Replacement (HMR) exchanges, adds, or removes modules while an application is running, without a full reload. This can significantly speed up development in a few ways:
- Retain application state which is lost during a full reload.
 - Save valuable development time by only updating what's changed.
 - Instantly update the browser when modifications are made to CSS/JS in the source code, which is almost comparable to changing styles directly in the browser's dev tools.
 - HMR is an opt-in feature that only affects modules containing HMR code.