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You can migrate a production React single-page application to the Next.js App Router without rewriting the whole app. The lower-risk path in Next.js’s migration guidance is to get the existing app running in a Next.js shell first, retain its client-side behavior and router, then move routes and rendering capabilities over in deliberate slices. Hydration errors become a central concern when you adopt server rendering: the browser’s first render must match the HTML produced before it.
How do I migrate a React SPA to Next.js App Router?
Separate the migration into two jobs: first, make the existing application run inside Next.js; later, adopt the App Router’s server-first model where it helps. The Vite and Create React App migration guidance both describe a client-side starting point. Keeping the existing router initially can reduce the number of behaviors changing at once and makes it easier to isolate migration problems.
- Inventory the application. For each route or feature, note its routing behavior, browser API usage, data dependencies, authentication needs, and whether it must render on the server. This is a planning checklist, not a prescribed Next.js checklist.
- Establish the Next.js shell. Bring the legacy application into a client-side entry point and keep its current router while you verify that the application starts and its key routes still behave as expected.
- Stabilize before changing the rendering contract. Resolve shell and routing issues before converting individual routes. That keeps problems caused by integration distinct from problems caused by server rendering or hydration.
- Migrate in slices. Move routes or features toward App Router conventions as the team is ready to handle their data, rendering, and client/server boundaries. Validate each slice against its route inventory.
- Choose the deployment mode deliberately. Decide whether the transitional app should be a static export or whether the application needs Next.js server-side capabilities. That choice affects which features are available, not just how the migration is packaged.
The official migration guidance presents incremental adoption as a path to App Router capabilities such as file-based routing, automatic code splitting, streaming server rendering, and React Server Components. These are capabilities, not guaranteed performance improvements; actual results depend on the application, and no before-and-after benchmark is established for a particular production SPA here.
What should stay client-side, and what changes with App Router?
App Router pages and layouts are Server Components by default. Server Components can produce the React Server Component payload used to reconcile the component tree. Client Components contain browser-side interactivity and are hydrated so their event handlers work.
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One important distinction for migration planning: a Client Component is not automatically excluded from server rendering. On an initial page load, Next.js can prerender Client Components to HTML, then hydrate them in the browser. On later navigations, Client Components render on the client. Consequently, adding 'use client' does not by itself preserve the old SPA’s client-only initial render.
Keep client boundaries narrow
'use client' marks a boundary in the module graph. Imports and descendants below that boundary become part of the client bundle. Put the directive around the interactive feature or browser-dependent area that needs it rather than at the top of a large tree by default. Suitable data and presentation work can remain in Server Components as the migration proceeds.
Use a client-only bridge only where needed
For a legacy component that fundamentally cannot run during prerendering, the migration guidance shows using next/dynamic with { ssr: false } to keep that selected component from being prerendered. This can help preserve a strictly client-side legacy entry point while integrating it into Next.js. Treat it as a targeted bridge, not as a reason to keep every future route client-only.
Static export or Next.js server features?
Create React App’s migration guidance describes output: 'export' as producing a static export. Static export can be useful during a staged migration, but it excludes server-side features. If the application needs server rendering or other server capabilities, remove that setting and make sure the deployment setup supports the features you intend to use.
| Decision area | Keep the SPA client-side initially | Adopt App Router capabilities incrementally |
|---|---|---|
| Migration risk | Preserves more existing behavior and follows the documented starting approach. | Introduces server/client boundaries and new routing or data patterns route by route. |
| Initial rendering | A legacy app can remain strictly client-side when prerendering is disabled for it. | Server-rendered HTML and hydration become part of the initial-load contract. |
| Routing | The existing router can remain in place during initial setup. | Moving routes to App Router uses its file-based routing and associated capabilities. |
| Server features | Static export does not provide server-side features. | Removing static export permits Next.js server features, subject to deployment setup. |
| Client JavaScript | The legacy client app remains client-heavy. | Server Components may reduce client-side work, but the outcome depends on the application; no measured result is established here. |
Why am I getting a hydration error?
Hydration is React’s process for attaching event handlers to server-generated HTML so that it becomes interactive. A mismatch occurs when the tree rendered for the browser’s first hydration render differs from the tree used to produce the server’s prerendered HTML. The mismatch is often a symptom of two environments producing different initial output, not a problem solved by hiding the warning.
- Invalid HTML nesting: for example, nested paragraph elements or an interactive element nested inside another of the same kind.
- Environment-dependent branches: rendering different markup based on a check such as
typeof window !== 'undefined'. - Browser-only APIs in render logic: reading
windoworlocalStoragebefore the initial render is complete. - Time-dependent output: calling
Date()during rendering can produce a different value on the server and in the browser. - External changes to the document: a browser extension can mutate the HTML, or an edge/CDN layer can modify the response. Next.js’s error guidance cites Cloudflare Auto Minify as an example of the latter.
- Styling integration: incorrect CSS-in-JS configuration can lead to different generated output.
How do I find the source of a hydration mismatch?
Work from the differing output toward its cause. This sequence applies the documented causes in a practical debugging order; it is not a required Next.js diagnostic procedure.
- Inspect the mismatch diff. Identify the element or text that differs, then trace it to the component responsible for that initial output.
- Check the generated markup. Look for invalid nesting, especially nested paragraphs or repeated interactive elements.
- Compare server and first-client render inputs. Search the render path for environment checks, browser storage or APIs, the current time, randomness, or other values that can vary between renders.
- Inspect style generation. If the component tree and inputs appear stable, verify that the CSS-in-JS integration is configured correctly.
- Check the delivered HTML. If the application’s output is correct before delivery, investigate browser extensions and any edge or CDN transformations that may alter the response.
How do I fix a hydration mismatch?
Choose a fix based on whether the initial value should be shared, browser-specific, or different by design.
Make shared initial content deterministic
If content should be identical on the server and in the browser, make both renders use the same initial value and produce the same markup. Correct invalid nesting and remove render-time conditions that cause the two environments to take different branches.
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Defer browser-only updates until after hydration
If a value is available only in the browser, render a stable initial state and update it in a useEffect. Effects run after hydration, so browser APIs such as localStorage can be read there without changing the initial server-versus-client comparison.
Disable prerendering for a browser-dependent component
If a specific component fundamentally depends on the browser and cannot produce server-compatible initial output, use a targeted dynamic import with ssr: false. This avoids prerendering that component, but shifts its initial rendering behavior and should not be applied broadly without a reason.
Reserve warning suppression for a narrow, unavoidable difference
suppressHydrationWarning is an escape hatch for a small difference such as a timestamp, not a general repair. It applies only one level deep, and React does not patch mismatched text content when it is set. It can therefore leave the differing text in place while concealing a symptom; it does not make a broader tree mismatch safe.
Why are timestamps and relative-time labels especially tricky?
A timestamp, relative-time label, or current-year value can change between prerendering and hydration simply because the two renders happen at different times. First decide when the value is meant to be calculated: as cached output, per request, or in the browser. Next.js’s rendering guidance treats those intents differently. For browser-driven updates, defer the change to a Client Component or an effect; do not use warning suppression as a blanket fix for clock-dependent UI.
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