feat: allow banner and footer to a function

which accepts `format` in the parameter

src/esbuild/index.ts

@@ -123,6 +123,15 @@ export async function runEsbuild(
     ...(options.esbuildPlugins || []),
   ]
 
+  const banner =
+    typeof options.banner === 'function'
+      ? options.banner({ format })
+      : options.banner
+  const footer =
+    typeof options.footer === 'function'
+      ? options.footer({ format })
+      : options.footer
+
   try {
     result = await esbuild({
       entryPoints: options.entry,
@@ -134,8 +143,8 @@ export async function runEsbuild(
       jsxFragment: options.jsxFragment,
       sourcemap: options.sourcemap ? 'external' : false,
       target: options.target,
-      footer: options.footer,
-      banner: options.banner,
+      banner,
+      footer,
       tsconfig: options.tsconfig,
       loader: {
         '.aac': 'file',

src/options.ts

@@ -16,6 +16,13 @@ export type DtsConfig = {
   footer?: string
 }
 
+export type BannerOrFooter =
+  | {
+      js?: string
+      css?: string
+    }
+  | ((ctx: { format: Format }) => { js?: string; css?: string } | undefined)
+
 /**
  * The options available in tsup.config.ts
  * Not all of them are available from CLI flags
@@ -106,8 +113,8 @@ export type Options = {
    * @see https://esbuild.github.io/api/#metafile
    */
   metafile?: boolean
-  footer?: BuildOptions['footer']
-  banner?: BuildOptions['banner']
+  footer?: BannerOrFooter
+  banner?: BannerOrFooter
   /**
    * Target platform
    * @default `node`