fix: add .js extensions to relative imports (#238)

[laazyj]

Problem

Since 4.0.5 the package is "type": "module", but the emitted .d.ts files still use extensionless relative specifiers (export { Graph } from './lib/graph'). Under moduleResolution node16/nodenext those are invalid, so a dependent gets:

node_modules/@dagrejs/graphlib/dist/types/index.d.ts(30,23): error TS2834:
Relative import paths need explicit file extensions in ECMAScript imports
when '--moduleResolution' is 'node16' or 'nodenext'.

The bug has a quieter second face that is arguably worse. With skipLibCheck: true — the common default — the error is silenced, but the declarations still fail to resolve and every graphlib export degrades to any. Type safety is gone at every call site with nothing on screen to say so.

Reported in #238. Only index.ts's type-only re-export of ./lib/types.js already carried an extension, so the convention was already half-applied in the repo.

Reproduction

A consumer project resolving @dagrejs/graphlib through node_modules and the exports map, moduleResolution: nodenext:

	before	after
skipLibCheck: false	4 × TS2834 on dist/types/index.d.ts	clean
skipLibCheck: true	no error, but const n: number = g.nodes() type-checks — Graph is any	correctly errors

Fix

1. Add .js to every relative specifier in lib/** and index.ts. Purely mechanical, 54 lines, one shape:

-import {Graph} from '../graph';
+import {Graph} from '../graph.js';

2. Compile tsconfig.build.json with nodenext (2 lines), so this cannot silently come back. That config already scopes itself to exactly the shipped surface (include: ["lib/**/*", "index.ts"], exclude: ["test/**/*"]), so the build now rejects a bad specifier at the source line rather than emitting declarations that break consumers:

lib/alg/dijkstra.ts(1,21): error TS2835: ... Did you mean '../graph.js'?

The emitted declarations are byte-identical to the previous bundler-resolved output — verified with diff -r. tsconfig.json stays on "bundler", so the extensionless imports throughout test/** and src/** are untouched and need no churn.

I looked at enforcing this with ESLint instead, but no installed plugin has such a rule (it would mean adding eslint-plugin-import), and it would be strictly weaker — import/extensions pattern-matches specifiers, whereas tsc actually resolves them.

3. Add test/dist-types.test.ts for the half that compiling this repo cannot cover: that a dependent resolving the package through node_modules and the "exports" map gets working types. That wiring can regress independently, as #233 showed. It builds declarations into a temp package next to a copy of the real package.json, then type-checks a consumer against it under nodenext with skipLibCheck off. The @ts-expect-error on a deliberate misuse is what catches the degrade-to-any case.

The test is hermetic: it builds from current source rather than reading the committed dist/, so it leaves the working tree alone and cannot pass on a stale artifact.

Backward compatibility

The change is additive — extensionless specifiers were valid in only some resolution modes, .js resolves in all of them. Verified with a consumer matrix (skipLibCheck off, plus a @ts-expect-error proving types genuinely resolve rather than falling back to any):

moduleResolution	result
node (node10), commonjs and esnext	pass
bundler	pass
node16, nodenext	pass
classic	fails — identically before and after, as it cannot resolve node_modules at all

Across TypeScript 4.0.8, 4.5.5, 5.0.4 and 5.9.3 under node10 resolution: all pass.

Runtime is unaffected, since esbuild inlines every relative import — 0 relative specifiers survive into any bundle. The unminified dist/graphlib.js rebuilt from this branch differs from the committed one by only the pending 4.0.5 → 4.0.6-pre version string, i.e. no logic change. CJS require(), ESM import and both IIFE bundles smoke-test working.

Validation

• Reproduced on a clean consumer before fixing; both faces of the failure confirmed gone after.
• Each guard checked to be load-bearing: reverting a single specifier fails the new test and the nodenext build.
• make lint, make build, make test all pass — 259 tests, 21 suites.

Note on dist/

dist/ is not included here. The history shows it is regenerated by the maintainer in the release build (prepublishOnly runs npm run build), and contributor fixes such as #235 do not touch it; #237 only did because its fix was a dist rename. The committed dist/types therefore still carries the bug until the next release build regenerates it. Happy to commit the rebuilt dist/ instead if you would prefer it in the PR — note that rebuilding also picks up the pending 4.0.5 → 4.0.6-pre version string, since the committed bundles predate that bump.

Closes #238