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Handling "Build Directory Not Writeable" Error in Next.js

Why This Error Occurred​

The "Build Directory Not Writeable" error usually occurs when the file system does not permit writing to the designated directory. A common scenario for this error is when you initiate a custom server in development mode on a production server.

These production servers often disallow writing to the filesystem after your application is built, causing this error.

Possible Ways to Fix It​

If you're deploying a custom server with a server file (let's assume it's named server.js), you should modify the scripts key in your package.json to the following:

{
  "scripts": {
    "dev": "node server.js",
    "build": "next build",
    "start": "NODE_ENV=production node server.js"
  }
}

Ensure that your custom server starts Next.js in production mode when NODE_ENV is set to production:

const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })