diff --git a/docs/guide/build.md b/docs/guide/build.md
index e2e18b2f5b5798..d45fe5a24168e8 100644
--- a/docs/guide/build.md
+++ b/docs/guide/build.md
@@ -108,7 +108,8 @@ module.exports = {
   build: {
     lib: {
       entry: path.resolve(__dirname, 'lib/main.js'),
-      name: 'MyLib'
+      name: 'MyLib',
+      fileName: format => `my-lib.${format}.js`  
     },
     rollupOptions: {
       // make sure to externalize deps that shouldn't be bundled
diff --git a/packages/vite/src/node/build.ts b/packages/vite/src/node/build.ts
index 276f47f2fa0301..65eae08f5d5cae 100644
--- a/packages/vite/src/node/build.ts
+++ b/packages/vite/src/node/build.ts
@@ -15,7 +15,8 @@ import Rollup, {
   GetModuleInfo,
   WatcherOptions,
   RollupWatcher,
-  RollupError
+  RollupError,
+  ModuleFormat
 } from 'rollup'
 import { buildReporterPlugin } from './plugins/reporter'
 import { buildHtmlPlugin } from './plugins/html'
@@ -189,7 +190,7 @@ export interface LibraryOptions {
   entry: string
   name?: string
   formats?: LibraryFormats[]
-  fileName?: string
+  fileName?: string | ((format?: ModuleFormat) => string)
 }
 
 export type LibraryFormats = 'es' | 'cjs' | 'umd' | 'iife'
@@ -414,7 +415,9 @@ async function doBuild(
         entryFileNames: ssr
           ? `[name].js`
           : libOptions
-          ? `${libOptions.fileName || pkgName}.${output.format || `es`}.js`
+          ? typeof libOptions.fileName === 'function'
+            ? libOptions.fileName(output.format)
+            : `${libOptions.fileName || pkgName}.${output.format || `es`}.js`
           : path.posix.join(options.assetsDir, `[name].[hash].js`),
         chunkFileNames: libOptions
           ? `[name].js`