diff --git a/packages/vite/src/node/__tests__/build.spec.ts b/packages/vite/src/node/__tests__/build.spec.ts
new file mode 100644
index 00000000000000..15bca18c91bfc4
--- /dev/null
+++ b/packages/vite/src/node/__tests__/build.spec.ts
@@ -0,0 +1,32 @@
+import { resolveLibFilename } from '../build'
+
+describe('resolveLibFilename', () => {
+  test('custom filename function', () => {
+    const filename = resolveLibFilename(
+      {
+        fileName: (format) => `custom-filename-function.${format}.js`,
+        entry: 'mylib.js'
+      },
+      'es',
+      'mylib'
+    )
+
+    expect(filename).toBe('custom-filename-function.es.js')
+  })
+
+  test('custom filename string', () => {
+    const filename = resolveLibFilename(
+      { fileName: 'custom-filename', entry: 'mylib.js' },
+      'es',
+      'mylib'
+    )
+
+    expect(filename).toBe('custom-filename.es.js')
+  })
+
+  test('package name as filename', () => {
+    const filename = resolveLibFilename({ entry: 'mylib.js' }, 'es', 'mylib')
+
+    expect(filename).toBe('mylib.es.js')
+  })
+})
diff --git a/packages/vite/src/node/build.ts b/packages/vite/src/node/build.ts
index 65eae08f5d5cae..7461c9fbf865a3 100644
--- a/packages/vite/src/node/build.ts
+++ b/packages/vite/src/node/build.ts
@@ -415,9 +415,7 @@ async function doBuild(
         entryFileNames: ssr
           ? `[name].js`
           : libOptions
-          ? typeof libOptions.fileName === 'function'
-            ? libOptions.fileName(output.format)
-            : `${libOptions.fileName || pkgName}.${output.format || `es`}.js`
+          ? resolveLibFilename(libOptions, output.format, pkgName)
           : path.posix.join(options.assetsDir, `[name].[hash].js`),
         chunkFileNames: libOptions
           ? `[name].js`
@@ -613,6 +611,16 @@ function staticImportedByEntry(
   return someImporterIs
 }
 
+export function resolveLibFilename(
+  libOptions: LibraryOptions,
+  format: ModuleFormat | undefined,
+  pkgName: string
+): string {
+  return typeof libOptions.fileName === 'function'
+    ? libOptions.fileName(format)
+    : `${libOptions.fileName || pkgName}.${format || `es`}.js`
+}
+
 function resolveBuildOutputs(
   outputs: OutputOptions | OutputOptions[] | undefined,
   libOptions: LibraryOptions | false,