`checkJs` skips `.mjs`/`.cjs` beside a `.d.mts`/`.d.cts`, but not `.js` beside a `.d.ts`

[ljharb]

🔎 Search Terms

checkJs, allowJs, .d.mts, .d.cts, declaration file adjacent to implementation, file silently dropped from program, extension priority

🕗 Version & Regression Information

Reproduces identically on 5.9.3, 6.0.3, and 7.1.0-dev.20260916.1. Long-standing, not a regression.

⏯ Playground Link

Not applicable: needs two sibling files on disk plus moduleResolution: nodenext.

💻 Code

Three directories, identical but for extensions. Same tsconfig.json in each; package.json has "type": "module" only for the .mjs case.

index.js / index.mjs / index.cjs:

export const n = 1;

const bad = null;
bad.a.b.c();

index.d.ts / index.d.mts / index.d.cts:

export declare const n: number;

{
	"compilerOptions": {
		"target": "esnext",
		"module": "nodenext",
		"moduleResolution": "nodenext",
		"allowJs": true,
		"checkJs": true,
		"strict": true,
		"noEmit": true
	}
}

🙁 Actual behavior

index.js   + index.d.ts    ->  error TS18047: 'bad' is possibly 'null'.
index.mjs  + index.d.mts   ->  (no output)
index.cjs  + index.d.cts   ->  (no output)

The same authoring relationship — a hand-written declaration file beside its implementation — produces opposite outcomes based only on the file extension.

In the .mjs/.cjs cases the implementation is not merely unchecked, it is absent from the program: --listFiles reports only index.d.mts. In the .js case --listFiles reports both index.d.ts and index.js.

🙂 Expected behavior

The three rows should agree. Either all of them check the implementation, or none do.

I'd expect the .js row to be the correct one — the declaration file describes the module for consumers, and checkJs still checks the implementation — but consistency matters more than which way it goes.

Additional information

#47796 reported the .mjs/.mts variant and was closed as "Working as Intended", on the rationale that the JS file is a build artifact of the TS file, and that "the compiler only automatically pulls in one copy of a file with the same basename (the most TS-y one)".

That rationale doesn't extend to this case:

• A .d.mts is not something a .mjs is emitted from, so the build-artifact framing doesn't apply. .d.ts + .js is the same relationship, and there the compiler does pull in both.
• Neither does the one-copy-per-basename rule, for the same reason — index.js and index.d.ts are both in the program.

The failure is silent, which is what makes it expensive: a package shipping .mjs/.cjs with adjacent hand-written declarations gets no checking of its implementation at all, while every sibling module without a declaration file is checked normally. tsc exits 0 and CI is green. I found this only by deliberately breaking a file and noticing the build still passed. Per #47796, VS Code does report these errors, so the editor and CI silently disagree.

#64098 is the same silent-drop shape from wildcard include (hasFileWithHigherPriorityExtension), if the two are related.

So, concretely:

1. Make the three rows consistent, or
2. if this is intended, document which sibling extensions suppress checking, and emit a diagnostic when an input file is dropped from the program for this reason.