feat(vite): update LibraryOptions and add docs

docs/config/index.md

@@ -602,10 +602,10 @@ createServer()
 
 ### build.lib
 
-- **Type:** `{ entry: string, name?: string, formats?: ('es' | 'cjs' | 'umd' | 'iife')[], fileName?: string }`
+- **Type:** `{ entry: string, name?: string, formats?: ('es' | 'cjs' | 'umd' | 'iife')[], fileName?: string | ((format: ModuleFormat) => string) }`
 - **Related:** [Library Mode](/guide/build#library-mode)
 
-  Build as a library. `entry` is required since the library cannot use HTML as entry. `name` is the exposed global variable and is required when `formats` includes `'umd'` or `'iife'`. Default `formats` are `['es', 'umd']`. `fileName` is the name of the package file output, default `fileName` is the name option of package.json
+  Build as a library. `entry` is required since the library cannot use HTML as entry. `name` is the exposed global variable and is required when `formats` includes `'umd'` or `'iife'`. Default `formats` are `['es', 'umd']`. `fileName` is the name of the package file output, default `fileName` is the name option of package.json, it can also be defined as function taking the `format` as an argument.
 
 ### build.manifest
 

packages/vite/src/node/build.ts

@@ -193,7 +193,7 @@ export interface LibraryOptions {
   entry: string
   name?: string
   formats?: LibraryFormats[]
-  fileName?: string | ((format?: ModuleFormat) => string)
+  fileName?: string | ((format: ModuleFormat) => string)
 }
 
 export type LibraryFormats = 'es' | 'cjs' | 'umd' | 'iife'
@@ -619,7 +619,7 @@ export function resolveLibFilename(
   format: ModuleFormat | undefined,
   pkgName: string
 ): string {
-  return typeof libOptions.fileName === 'function'
+  return typeof libOptions.fileName === 'function' && format
     ? libOptions.fileName(format)
     : `${libOptions.fileName || pkgName}.${format || `es`}.js`
 }