Add support for async fn / promise in next.config.js/.mjs

[timneutkens]

• Add support for async function / promise export in next.config.js/.mjs
• Update docs

Adds support for https://twitter.com/timneutkens/status/1486075973204422665

But also the simpler version:

module.exports = async () => {
  return {
    basePath: '/docs'
  }
}

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added

Documentation / Examples

• Make sure the linting passes by running yarn lint

[ijjk]

Failing test suites

Commit: 10272a8

test/integration/config-promise-error/test/index.test.js

• Promise in next config > should throw error when a promise is return on config

Expand output

● Promise in next config › should throw error when a promise is return on config

expect(received).toMatch(expected)

Expected pattern: /Error: > Promise returned in next config\. https:\/\//
Received string:  "warn  - The `target` config is deprecated and will be removed in a future version.
See more info here https://nextjs.org/docs/messages/deprecated-target-config
warn  - The Next.js plugin was not detected in your ESLint configuration. See https://nextjs.org/docs/basic-features/eslint#migrating-existing-config
info  - Checking validity of types...
info  - Creating an optimized production build...
info  - Compiled successfully
info  - Collecting page data...
info  - Generating static pages (0/3)
info  - Generating static pages (3/3)
info  - Finalizing page optimization...·
Page                                       Size     First Load JS
┌ ○ /                                      274 B          71.3 kB
└ ○ /404                                   195 B          71.2 kB
+ First Load JS shared by all              71 kB
  ├ chunks/framework-0209bd2e36019aae.js   42 kB
  ├ chunks/main-7bd9f272e2dcdbde.js        26.9 kB
  ├ chunks/pages/_app-26477814f4b13881.js  1.37 kB
  └ chunks/webpack-fd82975a6094609f.js     727 B·
○  (Static)  automatically rendered as static HTML (uses no initial props)·
"

  24 |     const { stderr, stdout } = await nextBuild(appDir, undefined, {
  25 |       stderr: true,
> 26 |       stdout: true,
     |                    ^
  27 |     })
  28 |
  29 |     expect(stderr + stdout).toMatch(

  at Object.<anonymous> (integration/config-promise-error/test/index.test.js:26:33)

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
buildDuration	15.5s	15.3s	-218ms
buildDurationCached	6s	6s	-17ms
nodeModulesSize	359 MB	359 MB	-152 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.167	3.184	⚠️ +0.02
/ avg req/sec	789.31	785.25	⚠️ -4.06
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.353	1.345	-0.01
/error-in-render avg req/sec	1847.94	1858.46	+10.52

Client Bundles (main, webpack, commons)

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	27.4 kB	27.4 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.1 kB	71.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	326 B	326 B	✓
dynamic-HASH.js gzip	2.57 kB	2.57 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	919 B	919 B	✓
image-HASH.js gzip	4.94 kB	4.94 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.17 kB	2.17 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.5 kB	14.5 kB	✓

Client Build Manifests

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
index.html gzip	532 B	532 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
buildDuration	18.8s	18.6s	-204ms
buildDurationCached	6s	6s	-66ms
nodeModulesSize	359 MB	359 MB	-152 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.188	3.189	0
/ avg req/sec	784.07	784.03	⚠️ -0.04
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.347	1.343	0
/error-in-render avg req/sec	1855.9	1861.87	+5.97

Client Bundles (main, webpack, commons)

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.1 kB	42.1 kB	✓
main-HASH.js gzip	27.4 kB	27.4 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.2 kB	71.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.56 kB	2.56 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	911 B	911 B	✓
image-HASH.js gzip	4.97 kB	4.97 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.2 kB	2.2 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.5 kB	14.5 kB	✓

Client Build Manifests

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	timneutkens/next.js add/promise-export	Change
index.html gzip	534 B	534 B	✓
link.html gzip	548 B	548 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 880656e