perf(deps): read each package.json once when resolving dep subpaths

src/features/deps.ts

@@ -185,9 +185,15 @@ export function DepsPlugin(
   tsdownBundle: TsdownBundle,
 ): Plugin {
   const deps = pkg && Array.from(getProductionDeps(pkg))
+  // Keyed by package.json path. Every subpath import of a package reads the
+  // same file, so read it once per build.
+  const hasExportsCache: HasExportsCache = new Map()
 
   return {
     name: 'tsdown:deps',
+    buildStart() {
+      hasExportsCache.clear()
+    },
     resolveId: {
       filter: [include(and(id(/^[^.]/), importerId(/./)))],
       async handler(id, importer, extraOptions) {
@@ -382,7 +388,8 @@ export function DepsPlugin(
       }
       if (RE_PACKAGE_SPECIFIER.test(id)) {
         const resolvedDep =
-          shouldResolveDepSubpath && (await resolveDepSubpath(id, resolve))
+          shouldResolveDepSubpath &&
+          (await resolveDepSubpath(id, resolve, hasExportsCache))
         return resolvedDep ? [true, resolvedDep] : true
       }
       const resolved = await resolve()
@@ -394,7 +401,8 @@ export function DepsPlugin(
     if (deps) {
       if (deps.includes(id) || deps.some((dep) => id.startsWith(`${dep}/`))) {
         const resolvedDep =
-          shouldResolveDepSubpath && (await resolveDepSubpath(id, resolve))
+          shouldResolveDepSubpath &&
+          (await resolveDepSubpath(id, resolve, hasExportsCache))
         return resolvedDep ? [true, resolvedDep] : true
       }
 
@@ -494,7 +502,24 @@ export function getTypesPackageName(id: string): string | undefined {
   return `@types/${name.replace(/^@/, '').replace('/', '__')}`
 }
 
-async function resolveDepSubpath(id: string, resolve: ResolveFn) {
+// `undefined` when the package.json can't be read or parsed
+type HasExportsCache = Map<string, Promise<boolean | undefined>>
+
+async function readHasExports(
+  packageJsonPath: string,
+): Promise<boolean | undefined> {
+  try {
+    return !!JSON.parse(await readFile(packageJsonPath, 'utf8')).exports
+  } catch {
+    return undefined
+  }
+}
+
+async function resolveDepSubpath(
+  id: string,
+  resolve: ResolveFn,
+  hasExportsCache: HasExportsCache,
+) {
   const parts = id.split('/')
   // ignore scope
   if (parts[0][0] === '@') parts.shift()
@@ -504,15 +529,13 @@ async function resolveDepSubpath(id: string, resolve: ResolveFn) {
   const resolved = await resolve()
   if (!resolved?.packageJsonPath) return
 
-  let pkgJson: Record<string, any>
-  try {
-    pkgJson = JSON.parse(await readFile(resolved.packageJsonPath, 'utf8'))
-  } catch {
-    return
+  let hasExports = hasExportsCache.get(resolved.packageJsonPath)
+  if (!hasExports) {
+    hasExports = readHasExports(resolved.packageJsonPath)
+    hasExportsCache.set(resolved.packageJsonPath, hasExports)
   }
-
-  // no `exports` field
-  if (pkgJson.exports) return
+  // only rewrite packages that have a readable package.json without `exports`
+  if ((await hasExports) !== false) return
 
   const parsed = parseNodeModulesPath(resolved.id)
   if (!parsed) return

tests/__snapshots__/resolve-dep-subpath-without-exports-field/each-package-is-checked-for-exports-separately.snap.md

@@ -0,0 +1,10 @@
+## index.mjs
+
+```mjs
+import { lt } from "my-dep/functions/lt.js";
+import { a } from "with-exports/a";
+import { gt } from "my-dep/functions/gt.js";
+import { b } from "with-exports/b";
+export { a, b, gt, lt };
+
+```

tests/e2e.test.ts

@@ -1486,6 +1486,44 @@ describe('resolve dep subpath without exports field', () => {
 
     expect(fileMap['index.mjs']).toContain('my-dep/folder/index.js')
   })
+
+  test('each package is checked for exports separately', async (context) => {
+    const { fileMap } = await testBuild({
+      context,
+      files: {
+        'node_modules/my-dep/package.json': JSON.stringify({
+          name: 'my-dep',
+          main: 'index.js',
+        }),
+        'node_modules/my-dep/functions/lt.js': `export const lt = () => {}`,
+        'node_modules/my-dep/functions/gt.js': `export const gt = () => {}`,
+        'node_modules/with-exports/package.json': JSON.stringify({
+          name: 'with-exports',
+          exports: { './*': './dist/*.js' },
+        }),
+        'node_modules/with-exports/dist/a.js': `export const a = 1`,
+        'node_modules/with-exports/dist/b.js': `export const b = 2`,
+        'index.ts': `export * from './one'
+export * from './two'`,
+        'one.ts': `export { lt } from 'my-dep/functions/lt'
+export { a } from 'with-exports/a'`,
+        'two.ts': `export { gt } from 'my-dep/functions/gt'
+export { b } from 'with-exports/b'`,
+        'package.json': JSON.stringify({
+          name: 'test-pkg',
+          dependencies: { 'my-dep': '^1.0.0', 'with-exports': '^1.0.0' },
+        }),
+      },
+      options: {
+        deps: { resolveDepSubpath: true },
+      },
+    })
+
+    expect(fileMap['index.mjs']).toContain('my-dep/functions/lt.js')
+    expect(fileMap['index.mjs']).toContain('my-dep/functions/gt.js')
+    expect(fileMap['index.mjs']).toMatch(/from ["']with-exports\/a["']/)
+    expect(fileMap['index.mjs']).toMatch(/from ["']with-exports\/b["']/)
+  })
 })
 
 describe('neverBundle: true', () => {