test: ENG-03 share behavior fixtures across source and releases

test/helpers.test.js

@@ -0,0 +1,28 @@
+import assert from 'node:assert/strict'
+import fs from 'node:fs'
+import path from 'node:path'
+// eslint-disable-next-line test/no-import-node-test -- Verify actual JS/TS loading used by module regressions.
+import { test } from 'node:test'
+import { loadModules, workspace } from './helpers/load.js'
+
+test('Module fixtures load JS and TS without silently selecting an obsolete sibling', async () => {
+  const parent = path.join(workspace, 'refactor/.cache')
+  fs.mkdirSync(parent, { recursive: true })
+  const directory = fs.mkdtempSync(path.join(parent, 'unit-loader-'))
+  try {
+    const ts = path.join(directory, 'double.ts')
+    const js = path.join(directory, 'double.js')
+    fs.writeFileSync(ts, 'export default (value: number): number => value * 2')
+    assert.equal((await loadModules({ double: 'double' }, directory)).double(3), 6)
+    fs.writeFileSync(js, 'export default value => value * 2')
+    await assert.rejects(loadModules({ double: 'double' }, directory), /exactly one JS\/TS module/)
+    fs.unlinkSync(ts)
+    assert.equal((await loadModules({ double: 'double' }, directory)).double(3), 6)
+    assert(!fs.existsSync(path.join(directory, 'dist')))
+  }
+  finally {
+    const relative = path.relative(parent, directory)
+    assert(relative.startsWith('unit-loader-') && !relative.includes(path.sep))
+    fs.rmSync(directory, { recursive: true, force: true })
+  }
+})