next.js/test/development/watch-config-file/index.test.ts

import { nextTestSetup } from 'e2e-utils'
import { check } from 'next-test-utils'
import { join } from 'path'

describe('watch-config-file', () => {
  const { next } = nextTestSetup({
    files: join(__dirname, 'fixture'),
  })
  it('should output config file change', async () => {
    await check(async () => next.cliOutput, /ready/i)

    await check(async () => {
      await next.patchFile(
        'next.config.js',
        `
            console.log(${Date.now()})
            const nextConfig = {
              reactStrictMode: true,
              async redirects() {
                  return [
                    {
                      source: '/about',
                      destination: '/',
                      permanent: false,
                    },
                  ]
                },
            }
            module.exports = nextConfig`
      )
      return next.cliOutput
    }, /Found a change in next\.config\.js\. Restarting the server to apply the changes\.\.\./)

    await check(() => next.fetch('/about').then((res) => res.status), 200)
  })
})