nextjs - ✅(Solved) Fix experimental-build-mode compile outputs invalid code [5 pull requests, 1 comments, 2 participants]

nextjs2025-09-16 21:57:53

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vercel/next.js#83863•Fetched 2026-04-08 02:21:19

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AlessioGr

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AlessioGr

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Error Message

alessio@Alessio-Gravili's-MacBook-Pro-(16-inch,-2024)-M4-Max cissue % pnpm next build --experimental-build-mode generate ▲ Next.js 15.5.3

Environments: .env

Creating an optimized production build ... ✓ Collecting page data
Inlining static env ... Error occurred prerendering page "/". Read more: https://nextjs.org/docs/messages/prerender-error SyntaxError: Invalid left-hand side in assignment at <unknown> (.next/server/app/page.js:1169) Export encountered an error on /page: /, exiting the build. ⨯ Next.js build worker exited with code: 1 and signal: null

Fix Action

Fixed

Fixed by PR: templates: fix experimental-build-mode compile generating invalid code (https://github.com/payloadcms/payload/pull/13828)
Fixed by PR: chore(templates): fix experimental-build-mode compile generating inva… (https://github.com/dsm23/dsm23-next-payloadcms-postgres-template/pull/709)
Fixed by PR: chore(templates): fix experimental-build-mode compile generating inva… (https://github.com/dsm23/dsm23-next-payloadcms-monorepo-template/pull/364)
Fixed by PR: chore(templates): fix experimental-build-mode compile generating inva… (https://github.com/dsm23/dsm23-next-payloadcms-template/pull/884)
Fixed by PR: chore(templates): fix experimental-build-mode compile generating inva… (https://github.com/dsm23/dsm23-bun-next-payloadcms-template/pull/644)

PR fix notes

PR #13828: templates: fix experimental-build-mode compile generating invalid code

Repository: payloadcms/payload
Author: AlessioGr
State: closed | merged: True
Link: https://github.com/payloadcms/payload/pull/13828

Description (problem / solution / changelog)

When using pnpm next build --experimental-build-mode compile in our website template, Next.js outputs the following code for the getServerSideURL function:

If you then run pnpm next build --experimental-build-mode generate or pnpm next build --experimental-build-mode generate-env which inlines the env variable, it becomes the following code:

Suddenly, it no longer assigns to a variable but to a string, which is invalid JavaScript syntax, throwing the following error:

This PR works around this issue changing up the source code in getServerSideURL.

I have reported this issue to Next.js: https://github.com/vercel/next.js/issues/83863

To see the specific tasks where the Asana app for GitHub is being used, see below:
- https://app.asana.com/0/0/1211375615406662

Changed files

templates/website/src/utilities/getURL.ts (modified, +6/-11)
templates/with-vercel-website/src/utilities/getURL.ts (modified, +6/-11)

PR #709: chore(templates): fix experimental-build-mode compile generating inva…

Repository: dsm23/dsm23-next-payloadcms-postgres-template
Author: dsm23
State: closed | merged: True
Link: https://github.com/dsm23/dsm23-next-payloadcms-postgres-template/pull/709

Description (problem / solution / changelog)

…lid code

When using pnpm next build --experimental-build-mode compile in our website template, Next.js outputs the following code:

If you then run pnpm next build --experimental-build-mode generate or pnpm next build --experimental-build-mode generate-env which inlines the env variable, it becomes the following code:

Suddenly, it no longer assigns to a variable but to a string, which is invalid JavaScript syntax, throwing the following error:

This PR works around this issue changing up the source code in getServerSideURL.

I have reported this issue to Next.js: vercel/next.js#83863

https://github.com/payloadcms/payload/commit/ba2ab2f31d4ceb16c89722d6fb14f88528ed0f1a#diff-ff7aaa6e6d3eef2acc79c0e4fa312431d13f7025391b787aa2b652eac55093ef

Changed files

src/utilities/get-url/index.ts (modified, +6/-11)

PR #364: chore(templates): fix experimental-build-mode compile generating inva…

Repository: dsm23/dsm23-next-payloadcms-monorepo-template
Author: dsm23
State: closed | merged: True
Link: https://github.com/dsm23/dsm23-next-payloadcms-monorepo-template/pull/364

Description (problem / solution / changelog)

…lid code

When using pnpm next build --experimental-build-mode compile in our website template, Next.js outputs the following code:

If you then run pnpm next build --experimental-build-mode generate or pnpm next build --experimental-build-mode generate-env which inlines the env variable, it becomes the following code:

Suddenly, it no longer assigns to a variable but to a string, which is invalid JavaScript syntax, throwing the following error:

This PR works around this issue changing up the source code in getServerSideURL.

I have reported this issue to Next.js: vercel/next.js#83863

https://github.com/payloadcms/payload/commit/ba2ab2f31d4ceb16c89722d6fb14f88528ed0f1a#diff-ff7aaa6e6d3eef2acc79c0e4fa312431d13f7025391b787aa2b652eac55093ef

Changed files

apps/web/src/utilities/get-url/index.ts (modified, +6/-11)

PR #884: chore(templates): fix experimental-build-mode compile generating inva…

Repository: dsm23/dsm23-next-payloadcms-template
Author: dsm23
State: closed | merged: True
Link: https://github.com/dsm23/dsm23-next-payloadcms-template/pull/884

Description (problem / solution / changelog)

…lid code

When using pnpm next build --experimental-build-mode compile in our website template, Next.js outputs the following code:

If you then run pnpm next build --experimental-build-mode generate or pnpm next build --experimental-build-mode generate-env which inlines the env variable, it becomes the following code:

Suddenly, it no longer assigns to a variable but to a string, which is invalid JavaScript syntax, throwing the following error:

This PR works around this issue changing up the source code in getServerSideURL.

I have reported this issue to Next.js: vercel/next.js#83863

https://github.com/payloadcms/payload/commit/ba2ab2f31d4ceb16c89722d6fb14f88528ed0f1a#diff-ff7aaa6e6d3eef2acc79c0e4fa312431d13f7025391b787aa2b652eac55093ef

Changed files

src/utilities/get-url/index.ts (modified, +6/-11)

PR #644: chore(templates): fix experimental-build-mode compile generating inva…

Repository: dsm23/dsm23-bun-next-payloadcms-template
Author: dsm23
State: closed | merged: True
Link: https://github.com/dsm23/dsm23-bun-next-payloadcms-template/pull/644

Description (problem / solution / changelog)

…lid code

When using pnpm next build --experimental-build-mode compile in our website template, Next.js outputs the following code:

If you then run pnpm next build --experimental-build-mode generate or pnpm next build --experimental-build-mode generate-env which inlines the env variable, it becomes the following code:

Suddenly, it no longer assigns to a variable but to a string, which is invalid JavaScript syntax, throwing the following error:

This PR works around this issue changing up the source code in getServerSideURL.

I have reported this issue to Next.js: vercel/next.js#83863

https://github.com/payloadcms/payload/commit/ba2ab2f31d4ceb16c89722d6fb14f88528ed0f1a#diff-ff7aaa6e6d3eef2acc79c0e4fa312431d13f7025391b787aa2b652eac55093ef

Changed files

src/utilities/get-url/index.ts (modified, +6/-11)

Code Example

const getServerSideURL = () => {
  let url = process.env.NEXT_PUBLIC_SERVER_URL
  url = 'http://localhost:3000'

  return url
}

---

children: [
    "Get ",
    (process.env.NEXT_PUBLIC_SERVER_URL,
    (process.env.NEXT_PUBLIC_SERVER_URL =
      "http://localhost:3000"),
    process.env.NEXT_PUBLIC_SERVER_URL),
    " started by editing ",
    (0, d.jsx)("code", { children: "app/page.tsx" }),
    ".",
  ]

---

children: [
    "Get ",
    ("http://localhost:3000",
    ("http://localhost:3000" =
      "http://localhost:3000"),
    "http://localhost:3000"),
    " started by editing ",
    (0, d.jsx)("code", { children: "app/page.tsx" }),
    ".",
  ]

---

"http://localhost:3000" =
      "http://localhost:3000"

---

alessio@Alessio-Gravili's-MacBook-Pro-(16-inch,-2024)-M4-Max cissue % pnpm next build --experimental-build-mode generate
   ▲ Next.js 15.5.3
   - Environments: .env

   Creating an optimized production build ...
 ✓ Collecting page data    
   Inlining static env ...
Error occurred prerendering page "/". Read more: https://nextjs.org/docs/messages/prerender-error
SyntaxError: Invalid left-hand side in assignment
    at <unknown> (.next/server/app/page.js:1169)
Export encountered an error on /page: /, exiting the build.
 ⨯ Next.js build worker exited with code: 1 and signal: null

---

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Aug 11 21:15:09 PDT 2025; root:xnu-11417.140.69.701.11~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 16
Binaries:
  Node: 24.0.1
  npm: 10.9.0
  Yarn: N/A
  pnpm: 10.11.0
Relevant Packages:
  next: 15.5.3 // Latest available version is detected (15.5.3).
  eslint-config-next: 15.5.3
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

RAW_BUFFERClick to expand / collapse

Link to the code that reproduces this issue

https://github.com/AlessioGr/cissue

To Reproduce

Download my reproduction (or create a new app using create-next-app and apply this commit)
Create an .env file and add NEXT_PUBLIC_SERVER_URL='http://localhost:3000'
Run pnpm next build --experimental-build-mode compile. This will generate the invalid code
Run pnpm next build --experimental-build-mode generate - this will inline the environment variables and cause the invalid code to error

Important to note: running pnpm next build works fine.

The issue is the following function (used in a React component to make sure it gets executed during generate step):

const getServerSideURL = () => {
  let url = process.env.NEXT_PUBLIC_SERVER_URL
  url = 'http://localhost:3000'

  return url
}

Result after pnpm next build --experimental-build-mode compile:

children: [
    "Get ",
    (process.env.NEXT_PUBLIC_SERVER_URL,
    (process.env.NEXT_PUBLIC_SERVER_URL =
      "http://localhost:3000"),
    process.env.NEXT_PUBLIC_SERVER_URL),
    " started by editing ",
    (0, d.jsx)("code", { children: "app/page.tsx" }),
    ".",
  ]

Result after pnpm next build --experimental-build-mode generate:

children: [
    "Get ",
    ("http://localhost:3000",
    ("http://localhost:3000" =
      "http://localhost:3000"),
    "http://localhost:3000"),
    " started by editing ",
    (0, d.jsx)("code", { children: "app/page.tsx" }),
    ".",
  ]

Invalid code:

 "http://localhost:3000" =
      "http://localhost:3000"

=> error

alessio@Alessio-Gravili's-MacBook-Pro-(16-inch,-2024)-M4-Max cissue % pnpm next build --experimental-build-mode generate
   ▲ Next.js 15.5.3
   - Environments: .env

   Creating an optimized production build ...
 ✓ Collecting page data    
   Inlining static env ...
Error occurred prerendering page "/". Read more: https://nextjs.org/docs/messages/prerender-error
SyntaxError: Invalid left-hand side in assignment
    at <unknown> (.next/server/app/page.js:1169)
Export encountered an error on /page: /, exiting the build.
 ⨯ Next.js build worker exited with code: 1 and signal: null

Current vs. Expected behavior

Current: invalid code is generated, leading to error when executed Correct: no invalid code, no error

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Aug 11 21:15:09 PDT 2025; root:xnu-11417.140.69.701.11~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 16
Binaries:
  Node: 24.0.1
  npm: 10.9.0
  Yarn: N/A
  pnpm: 10.11.0
Relevant Packages:
  next: 15.5.3 // Latest available version is detected (15.5.3).
  eslint-config-next: 15.5.3
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

SWC, Output

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response

extent analysis

TL;DR

The issue can be fixed by modifying the getServerSideURL function to avoid reassigning a string literal, which is not a valid left-hand side in an assignment.

Guidance

The error occurs because the getServerSideURL function is being inlined and optimized, resulting in an invalid assignment operation.
To fix this, modify the getServerSideURL function to avoid reassigning the url variable, for example by using a conditional statement to set the url variable only once.
Verify that the fix works by running pnpm next build --experimental-build-mode generate and checking that the build completes without errors.
Consider refactoring the getServerSideURL function to make it more robust and less prone to optimization-related issues.

Example

const getServerSideURL = () => {
  let url = process.env.NEXT_PUBLIC_SERVER_URL;
  if (!url) {
    url = 'http://localhost:3000';
  }
  return url;
}

Notes

This fix assumes that the intention of the getServerSideURL function is to return a default URL if the NEXT_PUBLIC_SERVER_URL environment variable is not set.
If the intention is different, the fix may need to be adjusted accordingly.

Recommendation

Apply workaround: modify the getServerSideURL function to avoid reassigning a string literal, as shown in the example above. This should fix the error and allow the build to complete successfully.

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nextjs - ✅(Solved) Fix experimental-build-mode compile outputs invalid code [5 pull requests, 1 comments, 2 participants]

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Error Message

Fix Action

Fixed

PR fix notes

PR #13828: templates: fix experimental-build-mode compile generating invalid code

Description (problem / solution / changelog)

Changed files

PR #709: chore(templates): fix experimental-build-mode compile generating inva…

Description (problem / solution / changelog)

Changed files

PR #364: chore(templates): fix experimental-build-mode compile generating inva…

Description (problem / solution / changelog)

Changed files

PR #884: chore(templates): fix experimental-build-mode compile generating inva…

Description (problem / solution / changelog)

Changed files

PR #644: chore(templates): fix experimental-build-mode compile generating inva…

Description (problem / solution / changelog)

Changed files

Code Example

Link to the code that reproduces this issue

To Reproduce

Current vs. Expected behavior

Provide environment information

Which area(s) are affected? (Select all that apply)

Which stage(s) are affected? (Select all that apply)

Additional context

extent analysis

TL;DR

Guidance

Example

Notes

Recommendation

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