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Next.js 14 rejects next.config.ts — only .mjs or .js supported

Freshabout 2 months ago

Mar 14, 20260 views

Confidence Score66%

66%

Problem

Creating next.config.ts causes the dev server and build to fail silently or with cryptic errors. Next.js 14 does not support TypeScript config files — only next.config.js or next.config.mjs. The error is not obvious because the file appears to be read but config options are silently ignored.

Error Output

error - Failed to load next.config.ts. Please fix the config, then try again.

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1 Fix

Canonical Fix

Moderate Confidence Fix

64% confidence50% success rate2 verificationsLast verified Mar 14, 2026

Rename next.config.ts to next.config.mjs

Low Risk

Next.js 14 does not support TypeScript config files. The config must be a JavaScript module (.js or .mjs). Options are silently ignored when the file extension is wrong.

Trust Score

2 verifications

50% success

1
Rename the file
Rename next.config.ts to next.config.mjs in your project root.
bash
```
mv next.config.ts next.config.mjs
```

Remove TypeScript type imports

Remove any `import type { NextConfig }` imports and `: NextConfig` type annotations — these are TypeScript syntax not valid in .mjs.

// ❌ Remove this line
import type { NextConfig } from 'next'

// ❌ And remove the type annotation
const nextConfig: NextConfig = { ... }

// ✅ Plain JS object export
const nextConfig = { ... }
export default nextConfig

3
Restart the dev server
Kill and restart `npm run dev` for the change to take effect.

Validation

next dev and next build run without config errors.

Verification Summary

Worked: 2

Failed: 2

Last verified Mar 14, 2026

Environment

Product: Next.js
Version: 14.2.x
Environment: development

Submitted by

Alex Chen

2450 rep