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Abanoub Kerols
Abanoub Kerols

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Why React & Next.js Users Get `ChunkLoadError` After Deployment — And How to Fix It

#nextjs #react #webdev #javascript


Imagine this:

You have a React or Next.js application running in production.

You deploy a new version.

New visitors can use the website normally.

But some users who already had the website open suddenly see:

ChunkLoadError
Failed to load chunk
Failed to fetch dynamically imported module
Loading chunk failed
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Then they refresh the page...

And everything works again.

So what's happening?

Is the solution really to force every user to refresh?

No.

The refresh may recover the application, but it doesn't solve the underlying deployment problem.

The real issue is usually a version mismatch between the application running in the user's browser and the assets available on the server after deployment.

In this article, we'll understand why this happens and how production deployments should handle it.

Table of Contents

The Problem

Let's start with a simple scenario.

A user opens your application:

https://example.com
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At that moment, the production application is:

Version 1
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The browser downloads the HTML and JavaScript assets required by that version.

For example:

app-ABC123.js
dashboard-XYZ456.js
profile-111.js
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The user keeps the tab open.

After a few hours, you deploy a new version.

Now the server is running:

Version 2
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The new build generates different assets:

app-DEF789.js
dashboard-QWE321.js
profile-222.js
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Now we have:

Browser
   |
   | Version 1
   | Old runtime
   | Old chunks
   |
   v
--------------------
      Server
--------------------
   |
   | Version 2
   | New runtime
   | New chunks
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This is where things can go wrong.

What Happens When a User Opens the Application?

A modern frontend application is usually not one giant JavaScript file.

The browser may load:

HTML
 |
 +-- main JavaScript
 |
 +-- framework/runtime
 |
 +-- feature chunks
 |
 +-- dynamically loaded chunks
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For example:

flowchart TD
    A[Browser requests HTML] --> B[Application Runtime]
    B --> C[Initial JavaScript]
    B --> D[Dashboard Chunk]
    B --> E[Profile Chunk]
    B --> F[Other Dynamic Chunks]

Some chunks may only be loaded when the user navigates to a particular feature.

For example:

User opens application
        |
        v
Loads main application
        |
        v
User opens Dashboard
        |
        v
Browser requests dashboard chunk
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This is one of the reasons modern frontend applications can be fast.

The browser doesn't necessarily download everything immediately.

What Changes During a Deployment?

Now imagine we change the Dashboard.

Version 1 contains:

dashboard-ABC123.js
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After rebuilding the application:

Version 2 contains:

dashboard-DEF456.js
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The new version is deployed.

If the deployment process removes the old asset:

dashboard-ABC123.js
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then an existing browser session may still expect it.

That's the key problem.

Why Does the Old User Break?

Let's follow the request.

The user opened Version 1 earlier.

Their browser is still running the old runtime.

The old runtime says:

"I need dashboard-ABC123.js"
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The browser requests:

GET /assets/dashboard-ABC123.js
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But the deployment has already deleted it.

The server responds:

404 Not Found
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The frontend may then produce:

ChunkLoadError
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or:

Failed to fetch dynamically imported module
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The sequence looks like this:

sequenceDiagram
    participant Browser
    participant Server

    Browser->>Server: Request old chunk
    Server-->>Browser: 404 Not Found
    Browser->>Browser: ChunkLoadError

The important point is:

The browser isn't necessarily broken. The deployment made the asset that its current runtime needs unavailable.

Why Does Refreshing Fix It?

This is the interesting part.

When the user refreshes the page, the browser requests the current application again.

Now the server returns Version 2.

The browser receives:

Version 2 HTML
       |
       v
Version 2 Runtime
       |
       v
Version 2 Chunks
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Everything matches.

So the application works.

That's why a refresh appears to "fix" the problem.

But the refresh is only recovering the user by replacing the old application version with the new one.

It doesn't solve the deployment strategy.

The Real Problem: Version Mismatch

We can summarize the problem as:

Old Runtime
     +
Missing/Incompatible Old Assets
     =
Chunk Loading Failure
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Or:

Browser Version
       !=
Available Server Assets
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This is the mental model I use when debugging these issues.

Understanding JavaScript Chunks

Modern frontend build systems use techniques such as:

  • Code splitting
  • Lazy loading
  • Dynamic imports
  • Route-based splitting

Instead of generating:

application.js
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the build may produce:

main-ABC123.js
dashboard-DEF456.js
profile-GHI789.js
settings-JKL012.js
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The browser loads the chunks it needs.

For example:

const Dashboard = lazy(() => import("./Dashboard"));
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The exact implementation differs between React, Next.js, bundlers, and deployment environments, but the important concept is the same:

The runtime needs to know where to find the correct assets for its build.

If those assets disappear during deployment, existing sessions can fail.

The Real Solution

So what should we do?

Not:

Deploy
   ↓
Delete everything old
   ↓
Hope existing users refresh
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Instead, we want:

Build
  ↓
Create new release
  ↓
Publish new assets
  ↓
Validate release
  ↓
Switch traffic
  ↓
Keep old assets temporarily
  ↓
Cleanup later
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This leads to several important concepts:

  1. Immutable assets
  2. Versioned releases
  3. Atomic deployments
  4. Rolling deployments
  5. Correct caching
  6. Controlled recovery

Let's look at each one.

Immutable Assets

One of the most important ideas is immutable assets.

Suppose Version 1 generates:

app-A1B2C3.js
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Version 2 generates:

app-D4E5F6.js
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Instead of modifying the old file, we keep both:

/assets/

app-A1B2C3.js
app-D4E5F6.js
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Version 1 users can still request:

app-A1B2C3.js
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Version 2 users can request:

app-D4E5F6.js
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Both can coexist.

flowchart LR
    A[Old Users] --> B[Old Assets]
    C[New Users] --> D[New Assets]

    B --> E[Release 1]
    D --> F[Release 2]

This is much safer than replacing the contents of the same filename.

Why Do Asset Hashes Help?

Production builds commonly generate content-based filenames.

For example:

app-a82f91.js
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If the code changes:

app-b91d72.js
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The filename changes.

This gives us a very useful property:

If the filename contains the content hash, changing the content produces a different asset.

That means we can safely cache a particular asset for a long time because that exact file is not expected to change.

Versioned Releases

Another useful strategy is to treat every deployment as a separate release.

For example:

/releases/

release-101/
release-102/
release-103/
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Each release contains its own application build.

Conceptually:

release-101/
    build/
    assets/
    server/

release-102/
    build/
    assets/
    server/
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Then we have a pointer to the active release:

current -> release-102
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When we deploy:

current -> release-103
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The old release doesn't have to disappear immediately.

This gives existing users time to finish their current session.

Atomic Deployments

A dangerous deployment might look like:

Stop application
      ↓
Delete old build
      ↓
Copy new build
      ↓
Start application
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This creates a period where the application may be partially unavailable.

A safer approach is:

Build new release
      ↓
Upload new release
      ↓
Validate it
      ↓
Switch traffic
      ↓
Keep old release
      ↓
Cleanup later
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The switch between releases should be as atomic as possible.

For example:

flowchart TD
    A[Load Balancer / Nginx] --> B[Current Release]

    B --> C[Release 101]
    B --> D[Release 102]

    D --> E[New Release]

After validation:

Current → Release 102
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Then Release 101 can remain available until it is safe to remove.

Rolling Deployments

If your application runs multiple instances, another option is a rolling deployment.

Suppose we start with:

Server 1 → Version 1
Server 2 → Version 1
Server 3 → Version 1
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We deploy Version 2 gradually:

Server 1 → Version 2
Server 2 → Version 1
Server 3 → Version 1
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Then:

Server 1 → Version 2
Server 2 → Version 2
Server 3 → Version 1
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Finally:

Server 1 → Version 2
Server 2 → Version 2
Server 3 → Version 2
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Health checks and a load balancer can help ensure traffic is only sent to healthy instances.

This reduces the chance of taking the entire application offline during deployment.

Caching Strategy

Caching is another important part of this problem.

We normally have two very different categories of resources:

HTML
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and:

Hashed static assets
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They should not necessarily have the same caching strategy.

For hashed assets:

app-a82f91.js
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we can often use long-lived caching:

Cache-Control: public, max-age=31536000, immutable
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because if the application changes, the build generates another filename.

For example:

Old:
app-a82f91.js

New:
app-b91d72.js
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The old file doesn't need to be overwritten.

What About HTML?

HTML is different.

The HTML document references the assets belonging to a particular application version.

For example:

<script src="/assets/app-a82f91.js"></script>
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A new deployment may produce:

<script src="/assets/app-b91d72.js"></script>
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Therefore, caching HTML indefinitely can cause users to receive stale references.

A common high-level strategy is:

HTML
→ Revalidate more frequently

Hashed JS/CSS
→ Cache for a long time
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The exact configuration depends on your framework, CDN, reverse proxy, and hosting architecture.

What About Next.js?

Next.js uses generated build assets under paths such as:

/_next/static/
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A production deployment should treat the build as a coherent release.

The important principle is not:

"Never delete .next."

The important principle is:

Don't make assets required by an active application version disappear before that version is no longer needed.

How you achieve that depends on the deployment architecture.

For example, you might use:

Release 101
    |
    +-- .next
    +-- server
    +-- static assets

Release 102
    |
    +-- .next
    +-- server
    +-- static assets
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Then switch the active release after the new version is ready.

What About React?

The same general concept applies to React applications.

For example, a Vite production build may generate:

dist/

assets/
    index-A1B2C3.js
    Dashboard-D4E5F6.js
    index-G7H8I9.css
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After another build:

assets/
    index-J1K2L3.js
    Dashboard-M4N5O6.js
    index-P7Q8R9.css
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The deployment should not blindly destroy assets that an existing application version may still require.

The exact files and runtime behavior depend on the build tool and architecture, but the deployment principle remains the same.

What About Automatic Refresh?

Now we can answer the original interview question.

Could we detect:

ChunkLoadError
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and reload the page?

Yes.

A recovery mechanism could conceptually do:

ChunkLoadError
      ↓
Detect error
      ↓
Reload once
      ↓
Load current release
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This can be useful.

But:

Automatic reload is a fallback, not the primary deployment solution.

If your deployment consistently deletes assets required by active sessions, adding window.location.reload() doesn't fix the architecture.

It only hides the symptom.

Why Blindly Reloading Can Be Dangerous

Imagine a user is filling out a form:

Name: John
Email: john@example.com
Phone: 01012345678
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Then a chunk-loading error occurs.

If your application automatically executes:

window.location.reload();
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the user may lose unsaved state.

There is another possible problem.

If the deployment is still broken:

ChunkLoadError
      ↓
Reload
      ↓
ChunkLoadError
      ↓
Reload
      ↓
ChunkLoadError
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You can create a reload loop.

So if automatic recovery is implemented, it should be controlled carefully.

A Production-Oriented Architecture

A simplified architecture could look like this:

flowchart TD
    U[Users] --> CDN[CDN / Nginx]
    CDN --> LB[Load Balancer]

    LB --> R1[Current Release]
    LB --> R2[Previous Release]

    R1 --> A1[New Immutable Assets]
    R2 --> A2[Old Immutable Assets]

    R2 --> C[Cleanup Later]

The key idea is that the old assets don't disappear immediately.

Eventually:

Old users finish their sessions
        ↓
Old release is no longer needed
        ↓
Old assets can be cleaned up
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A Complete Deployment Flow

A good mental model is:

Developer pushes code
        ↓
CI/CD builds application
        ↓
Create versioned release
        ↓
Generate hashed assets
        ↓
Publish assets
        ↓
Run tests / health checks
        ↓
Deploy new application version
        ↓
Switch traffic
        ↓
Keep old assets/release temporarily
        ↓
Users naturally move to new version
        ↓
Cleanup old release
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This approach is much more robust than:

Delete old files
      ↓
Copy new files
      ↓
Hope everyone refreshes
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Is This Only a Next.js Problem?

No.

You can encounter similar deployment problems in applications built with:

  • React
  • Next.js
  • Angular
  • Vue
  • Nuxt
  • Vite-based applications
  • Other applications using code splitting and dynamically loaded assets

The exact error message may differ.

But the underlying idea is often:

Client is running Version N
        +
Server/CDN is serving Version N+1
        +
Required old assets are unavailable
        =
Application failure
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The Interview Answer

If an interviewer asks:

"Why do users who already have the website open get errors after we deploy a new React/Next.js version?"

A strong answer could be:

"It's usually caused by a version mismatch between the application runtime already running in the user's browser and the assets available after the deployment.

An existing user may still be running the old JavaScript runtime, which references a chunk from the previous build. If that chunk was removed during deployment, the browser gets a 404 and the application can throw a ChunkLoadError or fail to load a dynamic module.

I wouldn't solve this simply by forcing users to refresh. I'd use immutable, versioned assets and an atomic or rolling deployment strategy. Old assets should remain available long enough for existing sessions to drain, while new users receive the new release.

I'd also configure caching appropriately: hashed static assets can be cached for a long time, while HTML needs a different caching strategy.

An automatic reload can be used as a fallback recovery mechanism, but it shouldn't be the primary deployment solution."

That answer demonstrates that you understand more than React itself.

You understand the relationship between:

Frontend
   +
Build System
   +
Browser
   +
Caching
   +
CDN
   +
Deployment
   +
Infrastructure
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Final Takeaway

The problem isn't simply:

"The user needs to refresh."

The deeper problem is:

A user can continue running an older version of a frontend application after the server has already deployed a newer version.

If the old runtime needs assets that the new deployment has removed, the application can fail.

The solution is to design deployments around version compatibility.

The key concepts are:

Immutable Assets
        +
Versioned Releases
        +
Atomic / Rolling Deployments
        +
Proper Cache Strategy
        +
Temporary Retention of Old Assets
        +
Controlled Recovery
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Once you understand this model, ChunkLoadError is no longer just a mysterious React or Next.js error.

It's a client/server deployment consistency problem.

And that's the real answer to the interview question.

One More Thing

The next time someone answers this interview question with:

"Just force the user to refresh."

ask:

"What happens to the JavaScript chunks that the existing browser session is already using?"

That question gets you much closer to the real problem.

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