diff --git a/types/rollup/index.d.ts b/types/rollup/index.d.ts
index 567dd148ae..fcd5f799c3 100644
--- a/types/rollup/index.d.ts
+++ b/types/rollup/index.d.ts
@@ -138,9 +138,9 @@ export interface Plugin {
 	 */
 	resolveId?(importee: string, importer: string | undefined): string | null | undefined | false | 0 | ''
 	/** A module transformer function */
-	transform?(this: TransformContext, source: string, id: string): string | null | undefined | { code: string, map: SourceMap }
+	transform?(this: TransformContext, source: string, id: string): TransformResult | Promise<TransformResult>
 	/** A bundle transformer function */
-	transformBundle?(source: string, options: { format: Format }): string | null | undefined | { code: string, map: SourceMap }
+	transformBundle?(source: string, options: { format: Format }): TransformResult | Promise<TransformResult>
 	/** Function hook called when bundle.generate() is being executed. */
 	ongenerate?(options: GenerateOptions, bundle: Bundle): void
 	/** Function hook called when bundle.write() is being executed, after the file has been written to disk. */
@@ -163,6 +163,8 @@ export interface TransformContext {
 	error(message: string | { message: string }, pos?: number | { line: number, column: number }): void
 }
 
+export type TransformResult = string | null | undefined | { code: string, map: SourceMap }
+
 /** Returns a Promise that resolves with a bundle */
 export function rollup(options: Options): Promise<Bundle>
 
diff --git a/types/rollup/rollup-tests.ts b/types/rollup/rollup-tests.ts
index b9a9eac5d7..e2b18d6109 100644
--- a/types/rollup/rollup-tests.ts
+++ b/types/rollup/rollup-tests.ts
@@ -21,6 +21,9 @@ const plugin: Plugin = {
             this.warn(`Prefer ' over " for strings`, indexOfQuote)
             return undefined
         }
+        if (id === 'foo') {
+            return Promise.resolve(source)
+        }
         return source
     },
     transformBundle(source, options) {
@@ -29,7 +32,9 @@ const plugin: Plugin = {
         } else if (options.format === 'cjs') {
             return null
         }
-
+        if (options.format !== 'es') {
+            return Promise.resolve(source)
+        }
         return undefined
     }
 }