fix: compute getPkgName only when used

packages/vite/src/node/__tests__/build.spec.ts

@@ -1,4 +1,5 @@
 import { resolveLibFilename } from '../build'
+import { resolve } from 'path'
 
 describe('resolveLibFilename', () => {
   test('custom filename function', () => {
@@ -8,25 +9,59 @@ describe('resolveLibFilename', () => {
         entry: 'mylib.js'
       },
       'es',
-      'mylib'
+      resolve(__dirname, 'packages/name')
     )
 
     expect(filename).toBe('custom-filename-function.es.js')
   })
 
   test('custom filename string', () => {
     const filename = resolveLibFilename(
-      { fileName: 'custom-filename', entry: 'mylib.js' },
+      {
+        fileName: 'custom-filename',
+        entry: 'mylib.js'
+      },
       'es',
-      'mylib'
+      resolve(__dirname, 'packages/name')
     )
 
     expect(filename).toBe('custom-filename.es.js')
   })
 
   test('package name as filename', () => {
-    const filename = resolveLibFilename({ entry: 'mylib.js' }, 'es', 'mylib')
+    const filename = resolveLibFilename(
+      {
+        entry: 'mylib.js'
+      },
+      'es',
+      resolve(__dirname, 'packages/name')
+    )
 
     expect(filename).toBe('mylib.es.js')
   })
+
+  test('custom filename and no package name', () => {
+    const filename = resolveLibFilename(
+      {
+        fileName: 'custom-filename',
+        entry: 'mylib.js'
+      },
+      'es',
+      resolve(__dirname, 'packages/noname')
+    )
+
+    expect(filename).toBe('custom-filename.es.js')
+  })
+
+  test('missing filename', () => {
+    expect(() => {
+      resolveLibFilename(
+        {
+          entry: 'mylib.js'
+        },
+        'es',
+        resolve(__dirname, 'packages/noname')
+      )
+    }).toThrow()
+  })
 })

packages/vite/src/node/__tests__/packages/name/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "mylib"
+}

packages/vite/src/node/__tests__/packages/noname/package.json

@@ -0,0 +1 @@
+{}

packages/vite/src/node/build.ts

@@ -422,8 +422,6 @@ async function doBuild(
   }
 
   try {
-    const pkgName = libOptions && getPkgName(config.root)
-
     const buildOutputOptions = (output: OutputOptions = {}): OutputOptions => {
       return {
         dir: outDir,
@@ -434,7 +432,7 @@ async function doBuild(
         entryFileNames: ssr
           ? `[name].js`
           : libOptions
-          ? resolveLibFilename(libOptions, output.format || 'es', pkgName)
+          ? resolveLibFilename(libOptions, output.format || 'es', config.root)
           : path.posix.join(options.assetsDir, `[name].[hash].js`),
         chunkFileNames: libOptions
           ? `[name].js`
@@ -576,9 +574,7 @@ function prepareOutDir(
 function getPkgName(root: string) {
   const { name } = JSON.parse(lookupFile(root, ['package.json']) || `{}`)
 
-  if (!name) throw new Error('no name found in package.json')
-
-  return name.startsWith('@') ? name.split('/')[1] : name
+  return name?.startsWith('@') ? name.split('/')[1] : name
 }
 
 function createMoveToVendorChunkFn(config: ResolvedConfig): GetManualChunk {
@@ -633,11 +629,20 @@ function staticImportedByEntry(
 export function resolveLibFilename(
   libOptions: LibraryOptions,
   format: ModuleFormat,
-  pkgName: string
+  root: string
 ): string {
-  return typeof libOptions.fileName === 'function'
-    ? libOptions.fileName(format)
-    : `${libOptions.fileName || pkgName}.${format}.js`
+  if (typeof libOptions.fileName === 'function') {
+    return libOptions.fileName(format)
+  }
+
+  const name = libOptions.fileName || getPkgName(root)
+
+  if (!name)
+    throw new Error(
+      'Name in package.json is required if option "build.lib.fileName" is not provided.'
+    )
+
+  return `${name}.${format}.js`
 }
 
 function resolveBuildOutputs(