Make sure externals is an array

[timneutkens]

Follow-up to #30299

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
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
buildDuration	15.8s	16.6s	⚠️ +768ms
buildDurationCached	3.3s	3.4s	⚠️ +116ms
nodeModulesSize	207 MB	207 MB	⚠️ +86 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.728	2.756	⚠️ +0.03
/ avg req/sec	916.4	907.18	⚠️ -9.22
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.384	1.327	-0.06
/error-in-render avg req/sec	1805.86	1883.43	+77.57

Client Bundles (main, webpack, commons)

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28 kB	28 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	71.9 kB	71.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
_app-HASH.js gzip	1.23 kB	1.23 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 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	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	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/fix-externals-signature	Change
index.html gzip	534 B	534 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
buildDuration	13.9s	14.2s	⚠️ +331ms
buildDurationCached	3.3s	3.4s	⚠️ +59ms
nodeModulesSize	207 MB	207 MB	⚠️ +86 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.832	2.905	⚠️ +0.07
/ avg req/sec	882.71	860.69	⚠️ -22.02
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.238	1.298	⚠️ +0.06
/error-in-render avg req/sec	2019.04	1925.83	⚠️ -93.21

Client Bundles (main, webpack, commons)

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.2 kB	28.2 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	72.1 kB	72.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
_app-HASH.js gzip	1.22 kB	1.22 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	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 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	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	timneutkens/next.js add/fix-externals-signature	Change
index.html gzip	534 B	534 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 69233af