diff --git a/crates/oxc_isolated_declarations/src/function.rs b/crates/oxc_isolated_declarations/src/function.rs
index b92b9e0b1fb3cd..ca5c6d8a500c7d 100644
--- a/crates/oxc_isolated_declarations/src/function.rs
+++ b/crates/oxc_isolated_declarations/src/function.rs
@@ -26,8 +26,8 @@ impl<'a> IsolatedDeclarations<'a> {
                 func.r#type,
                 func.span,
                 self.ast.copy(&func.id),
-                func.generator,
-                func.r#async,
+                false,
+                false,
                 self.ast.copy(&func.this_param),
                 params,
                 None,
diff --git a/crates/oxc_isolated_declarations/src/inferrer.rs b/crates/oxc_isolated_declarations/src/inferrer.rs
index 39245637ec9a23..874c8bfb9c8781 100644
--- a/crates/oxc_isolated_declarations/src/inferrer.rs
+++ b/crates/oxc_isolated_declarations/src/inferrer.rs
@@ -120,10 +120,6 @@ impl<'a> IsolatedDeclarations<'a> {
             return None;
         }
 
-        if function.r#async {
-            return None;
-        }
-
         if function.expression {
             if let Some(Statement::ExpressionStatement(stmt)) = function.body.statements.first() {
                 return self
diff --git a/crates/oxc_isolated_declarations/tests/fixtures/async-function.ts b/crates/oxc_isolated_declarations/tests/fixtures/async-function.ts
new file mode 100644
index 00000000000000..3927f10d1e10ed
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/fixtures/async-function.ts
@@ -0,0 +1,16 @@
+// Correct
+async function asyncFunctionGood(): Promise<number> {}
+const asyncFunctionGoo2 = async (): Promise<number> => {
+  return Promise.resolve(0);
+}
+
+
+// Need to explicit return type for async functions
+// Incorrect
+async function asyncFunction() {
+  return 42;
+}
+
+const asyncFunction2 = async () => {
+  return "Hello, World!";
+}
\ No newline at end of file
diff --git a/crates/oxc_isolated_declarations/tests/fixtures/generator.ts b/crates/oxc_isolated_declarations/tests/fixtures/generator.ts
new file mode 100644
index 00000000000000..c09739a260a9ea
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/fixtures/generator.ts
@@ -0,0 +1,10 @@
+// Correct
+function *generatorGood(): Generator<number> {}
+
+
+// Need to explicit return type for async functions
+// Incorrect
+function *generatorGoodBad() {
+  yield 50;
+  return 42;
+}
\ No newline at end of file
diff --git a/crates/oxc_isolated_declarations/tests/snapshots/async-function.snap b/crates/oxc_isolated_declarations/tests/snapshots/async-function.snap
new file mode 100644
index 00000000000000..3af230361db6d8
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/snapshots/async-function.snap
@@ -0,0 +1,31 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/async-function.ts
+---
+==================== .D.TS ====================
+
+declare function asyncFunctionGood(): Promise<number>;
+declare const asyncFunctionGoo2: () => Promise<number>;
+declare function asyncFunction();
+declare const asyncFunction2: unknown;
+
+
+==================== Errors ====================
+
+  x TS9007: Function must have an explicit return type annotation with
+  | --isolatedDeclarations.
+    ,-[10:16]
+  9 | // Incorrect
+ 10 | async function asyncFunction() {
+    :                ^^^^^^^^^^^^^
+ 11 |   return 42;
+    `----
+
+  x TS9007: Function must have an explicit return type annotation with
+  | --isolatedDeclarations.
+    ,-[14:30]
+ 13 | 
+ 14 | const asyncFunction2 = async () => {
+    :                              ^^^^^^^
+ 15 |   return "Hello, World!";
+    `----
diff --git a/crates/oxc_isolated_declarations/tests/snapshots/generator.snap b/crates/oxc_isolated_declarations/tests/snapshots/generator.snap
new file mode 100644
index 00000000000000..b72acbc6c276c2
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/snapshots/generator.snap
@@ -0,0 +1,20 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/generator.ts
+---
+==================== .D.TS ====================
+
+declare function generatorGood(): Generator<number>;
+declare function generatorGoodBad();
+
+
+==================== Errors ====================
+
+  x TS9007: Function must have an explicit return type annotation with
+  | --isolatedDeclarations.
+   ,-[7:11]
+ 6 | // Incorrect
+ 7 | function *generatorGoodBad() {
+   :           ^^^^^^^^^^^^^^^^
+ 8 |   yield 50;
+   `----