Tooling: Add .mts and .cts support

[manzoorwanijk]

What?

See #83033, #82731 (comment), #83066 (comment)

Teaches this repository's tooling to accept .mts and .cts files: formatting, linting, type checking, test discovery and the build.

Why?

Node runs both through type stripping with no build step, but no tool here matched them, so the first such file would have been skipped silently by every check. #83033 needs .mts for the scripts leaving bin/.

How?

Wherever a glob or regex already matched .ts, it now also matches .mts and .cts. {js,jsx}-only patterns are untouched. One commit per tool.

• ESLint's flat-config discovery reads only .js, .mjs and .cjs, so both are registered before the parser and rules can apply. The repeated [tj]s?(x) globs become named constants.
• @wordpress/build rewrites a local import to the emitted .mjs/.cjs, and esbuild never tries .mts for an extensionless import. Both dropped .mts silently, producing output resolving to nothing rather than a build error.
• tools/monorepo/tsconfig/node.base.json is the preset from Retire bin/: move the last four files into the workspaces that own them #83033, adopted by tools/pr-meta, and adds the erasableSyntaxOnly it lacked.
• wp-scripts format had no test; one now covers every extension it globs.

Follow-up: 35 package and route tsconfig.json files spell out their own include. A .mts there would build and validate but escape npm run typecheck. None exists yet, so that sweep gets its own PR.

Testing Instructions

1. npm run lint:js, npm run typecheck, npm run lint:tsconfig, npm run test:unit:conventions and npm run build pass.
2. npm run test:unit -- packages/scripts/scripts/test/format.js passes, and fails on trunk's list.

Use of AI Tools

Authored with Claude Code, reviewed by Codex and by me.

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Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>
Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>

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[github-actions]

🤖 PR meta 🤖

📦 Bundle size

Size Change: 0 B

Total Size: 8.21 MB

_{87fa779 Run}

⚡ Performance

Show the results

Client side metrics exclude the server response time.

front-end-block-theme

Metric	21fa3a4	trunk	% Change
timeToFirstByte	53.9 ms +7.98% -1.76%	54.8 ms +11.77% -3.74%	-1.64%
largestContentfulPaint	96 ms +4.17% -8.33%	94 ms +4.26% -10.64%	2.13%
lcpMinusTtfb	38.35 ms +11.86% -11.21%	32.8 ms +26.37% -3.35%	16.92%
wpBeforeTemplate	26.41 ms +14.65% -1.33%	26.47 ms +20.82% -1.51%	-0.23%
wpTemplate	23.02 ms +6.86% -1.52%	23.66 ms +3.3% -3.93%	-2.7%
wpTotal	49.96 ms +8.97% -1.62%	50.98 ms +11.24% -4.12%	-2%
wpMemoryUsage	7.53 MB +0% -0%	7.49 MB +0% -0%	0.47%
wpDbQueries	17 +0% -0%	17 +0% -0%	0%

front-end-classic-theme

Metric	21fa3a4	trunk	% Change
timeToFirstByte	43.45 ms +8.17% -1.15%	43.5 ms +5.17% -0.46%	-0.11%
largestContentfulPaint	100 ms +2% -4%	96 ms +6.25% -0%	4.17%
lcpMinusTtfb	53.85 ms +4.27% -1.49%	52.7 ms +7.12% -0.38%	2.18%
wpBeforeTemplate	23.23 ms +9.81% -1.59%	23.04 ms +1.87% -0.52%	0.82%
wpTemplate	17.29 ms +1.91% -2.14%	17.43 ms +2.41% -1.95%	-0.8%
wpTotal	40.41 ms +8.31% -0.99%	40.5 ms +5.38% -0.84%	-0.22%
wpMemoryUsage	6.15 MB +0% -0%	6.11 MB +0% -0%	0.6%
wpDbQueries	14 +0% -0%	14 +0% -0%	0%

media-processing

Metric	21fa3a4	trunk	% Change
mediaProcessingJpeg	412.85 ms +1.62% -0.72%	407.62 ms +1.1% -0.14%	1.28%
mediaProcessingAvif	6208.14 ms +0.2% -0.19%	6162.98 ms +0.31% -0.38%	0.73%
mediaProcessingJpegToAvif	4313.07 ms +0.26% -0.02%	4315.88 ms +0.52% -0.13%	-0.07%

media-upload

Metric	21fa3a4	trunk	% Change
jpegUploadProcessing	1458.84 ms +33.7% -3.33%	1402.73 ms +1.47% -0.44%	4%
pngUploadProcessing	197.79 ms +15.67% -1.13%	193.92 ms +8.73% -2.23%	2%
largeJpegUploadProcessing	1394.74 ms +0.52% -0.51%	1400.9 ms +0.51% -0.49%	-0.44%
multipleImageUploadProcessing	1534.57 ms +2% -1.61%	1564.65 ms +1.63% -0.74%	-1.92%

post-editor

Metric	21fa3a4	trunk	% Change
serverResponse	526.4 ms +3.49% -9.01%	494.67 ms +9.17% -5.12%	6.41%
firstPaint	218.38 ms +5.85% -7.27%	220.05 ms +22.67% -12.32%	-0.76%
domContentLoaded	1121.12 ms +2.23% -1.77%	1094.34 ms +1.22% -0.26%	2.45%
loaded	1122.5 ms +2.22% -1.76%	1096.74 ms +1.1% -0.37%	2.35%
firstContentfulPaint	464.31 ms +3.47% -4.02%	449.85 ms +2.29% -3.12%	3.21%
firstBlock	3325.43 ms +1% -0.5%	3267.68 ms +0.41% -0.73%	1.77%
type	18.41 ms +12.76% -4.07%	20.06 ms +2.54% -6.48%	-8.23%
typeWithoutInspector	18.77 ms +4.05% -3.09%	19.85 ms +3.88% -6.25%	-5.44%
typeWithTopToolbar	24.52 ms +5.46% -3.83%	25.36 ms +2.44% -7.33%	-3.31%
typeContainer	8.33 ms +5.88% -4.08%	9.07 ms +7.94% -3.53%	-8.16%
focus	74.53 ms +4.07% -2.42%	77.2 ms +5.12% -10.87%	-3.46%
firstFocus	206.67 ms +0% -0%	210.6 ms +0% -0%	-1.87%
selectAll	507.44 ms +5.22% -1.87%	514.4 ms +5.51% -0.63%	-1.35%
listViewOpen	73.13 ms +4.17% -9.13%	74.68 ms +8.93% -7.19%	-2.08%
inserterOpen	23.18 ms +3.93% -11.17%	23.34 ms +3% -5.48%	-0.69%
inserterHover	2.9 ms +8.62% -11.38%	3.21 ms +18.69% -10.9%	-9.66%
inserterSearch	8.13 ms +4.06% -9.84%	7.63 ms +9.17% -6.42%	6.55%
loadPatterns	639.18 ms +6.33% -7.13%	642.56 ms +4.52% -2%	-0.53%
wpTotal	516.44 ms +3.41% -9.3%	485 ms +9.3% -5.31%	6.48%
wpMemoryUsage	13.04 MB +0% -0%	13.01 MB +0% -0%	0.29%
wpDbQueries	54 +0% -1.85%	54 +0% -1.85%	0%

site-editor

Metric	21fa3a4	trunk	% Change
serverResponse	501.83 ms +5.27% -4.27%	507.01 ms +1.34% -1.34%	-1.02%
firstPaint	252.05 ms +1.95% -6.04%	292.91 ms +58.68% -19.2%	-13.95%
domContentLoaded	1149.91 ms +1.06% -1.05%	1162.64 ms +2.03% -0.5%	-1.09%
loaded	1151.24 ms +1.05% -1.06%	1163.92 ms +2.03% -0.49%	-1.09%
firstContentfulPaint	457.68 ms +3.71% -0.68%	466.22 ms +4.21% -2.06%	-1.83%
firstBlock	4237 ms +1.1% -0.6%	4388.73 ms +1.53% -1.64%	-3.46%
type	21.77 ms +2.53% -7.44%	22.07 ms +6.52% -5.17%	-1.36%
navigate	124.53 ms +8.17% -8.21%	135.39 ms +7.33% -4.84%	-8.02%
loadPatterns	1398.1 ms +8.08% -7.89%	1457.06 ms +6.38% -5.22%	-4.05%
loadPages	1060.29 ms +1.17% -3.02%	1111.86 ms +2.72% -2.07%	-4.64%
wpTotal	492.5 ms +5.21% -4.51%	496.93 ms +1.4% -1.24%	-0.89%
wpMemoryUsage	12.02 MB +0% -0%	11.97 MB +0% -0%	0.41%
wpDbQueries	43 +2.33% -0%	43 +2.33% -0%	0%

_{87fa779 Run}

[jsnajdr]

There are many places that add the mts and cts extensions, but don't support mjs and cjs. That should be consistently updated.

For #83066 we'll also need a brand new test/unit/tsconfig.json file, there currently isn't one.

[jsnajdr]

Does this mean that ESLint was skipping .jsx files until now?

[manzoorwanijk]

Yes. wp-scripts lint-js was silently skipping .jsx in consumer projects. Flat config only discovers .js, .mjs and .cjs by default; recommended registers .ts/.tsx via its files entry, but nothing registered .jsx.

[manzoorwanijk]

There are many places that add the mts and cts extensions, but don't support mjs and cjs. That should be consistently updated.

Done in ffe2963. SOURCE_EXTENSIONS gained cjs, which @wordpress/build never globbed, and route and widget discovery follow it. Test discovery matches the module spellings too.

For #83066 we'll also need a brand new test/unit/tsconfig.json file, there currently isn't one.

I will leave that to the PR that adds a ts file there.

[jsnajdr]

There are type errors in route-utils.mjs, not sure what caused them. Otherwise I have no more feedback.

I will leave that to the PR that adds a ts file there.

Yes, that's acceptable, this PR does all the big work already.