Unlocking the Power of Next.js Hot Module Replacement (HMR)
In the fast-paced world of modern web development, efficiency and productivity are crucial. That’s why Next.js introduced Hot Module Replacement (HMR), also known as Fast Refresh. This powerful feature ensures that any code changes are quickly reflected in the browser without requiring a full page reload, enhancing your development experience.
What is Next.js HMR?
Fast Refresh is a Next.js feature designed for local development servers. It provides instantaneous feedback every time you edit a React component on the page, re-rendering only the components that have been updated. This means that most changes should be visible within a second, without affecting the current component state.
How Does HMR Work in Next.js?
Fast Refresh attempts to hot reload the updated components. If this isn’t possible, it performs a full reload of the entire page as a fallback plan. There are two actions that trigger Next.js hot reload:
- Editing a React component file: If you update the styles or Hooks in a component file, Fast Refresh will re-render the component.
- Editing a file containing exports used in React components: If you update a file with exports that aren’t React components themselves, but are used in React components, Fast Refresh will re-run that file along with any files that import it.
Common Next.js HMR Issues and How to Fix Them
Before diving into HMR issues, make sure you’re not experiencing a caching problem. Next.js builds the local server in the .next folder and sometimes fails to update it properly. As a general solution, follow these steps:
- Stop the local dev server
- Delete the
.nextfolder - Run the dev server again with
next dev
If this doesn’t solve the problem, you may be experiencing a real HMR issue. Let’s explore some common scenarios where Next.js hot reload doesn’t work as expected:
- Component local state is not preserved: In some cases, Next.js HMR might fail to preserve the component state. This can occur when you edit a class component or a functional component whose export is wrapped by a higher-order function that returns a class. The solution is to use only function components with Hooks.
- Next.js HMR is not disabled in production: Fast Refresh is disabled by default when not running Next.js on a local server. If you notice regular spikes associated with calls to
_next/webpack-hmrin your production application, check if Next.js HMR is enabled in production. - Updates to i18n translations are not rendered: If you add a new translation or update an existing one when developing locally, nothing will happen. Fast Refresh will not trigger, and the page will not reflect the update. To address this issue, add the following lines to your
pages/_app.jsxorapp/layout.jsxcomponent:
“`
import { useTranslation } from ‘next-i18next’;
const { i18n } = useTranslation();
i18n.reloadResources();
“
* Updates to next.config.js do not propagate: If you editwhen running your application locally, Fast Refresh will ignore the updates. This is becausenext.config.jsis not a React component or a file imported by a React component, so it doesn't meet the requirements to trigger a hot reload..env` files trigger a hot reload. However, if you’re using an older version, you’ll need to manually relaunch the development server to see the changes.
* Changes to.env files are not loaded: As of Next.js v12.3, changes to
* Experiencing FOUCs when using middleware.js with the App Router: A flash of unstyled content (FOUC) can occur when using middleware.js with the App Router. To fix this issue, update Next.js to version 13.3 or higher.
* Cannot disable Next.js HMR: Unfortunately, there is no official way to disable Fast Refresh in Next.js. The community has found some workarounds, but most of them no longer work. Since disabling Fast Refresh is discouraged by the official team, you should not do it.
By understanding how Next.js HMR works and addressing common issues, you can unlock the full potential of this powerful feature and enhance your development experience.