From a118f2f3480e9481bcaa8aa9cd5861a3f8e56cd7 Mon Sep 17 00:00:00 2001
From: Nicholas Hulston <nicholas.hulston@datadoghq.com>
Date: Mon, 9 Jun 2025 18:49:28 -0400
Subject: [PATCH 1/3] resolve immediately when handler returns undefined

---
 src/utils/handler.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/utils/handler.ts b/src/utils/handler.ts
index 4874bbf8d..b7224c072 100644
--- a/src/utils/handler.ts
+++ b/src/utils/handler.ts
@@ -121,6 +121,9 @@ export function promisifiedHandler<TEvent, TResult>(handler: Handler<TEvent, TRe
     if (asyncProm !== undefined && typeof asyncProm.then === "function") {
       // Mimics behaviour of lambda runtime, the first method of returning a result always wins.
       promise = Promise.race([callbackProm, asyncProm]);
+    } else if (asyncProm === undefined) {
+      // Handler returned undefined (also happens when missing return statement), resolve immediately
+      promise = Promise.resolve(undefined);
     }
     return promise;
   };

From 148151e5d3d787ce3f6b67d95cc3ac60403dfc3d Mon Sep 17 00:00:00 2001
From: Nicholas Hulston <nicholas.hulston@datadoghq.com>
Date: Mon, 9 Jun 2025 19:03:57 -0400
Subject: [PATCH 2/3] only resolve immediately if handler doesn't return a
 callback

---
 src/utils/handler.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/utils/handler.ts b/src/utils/handler.ts
index b7224c072..61775f85c 100644
--- a/src/utils/handler.ts
+++ b/src/utils/handler.ts
@@ -121,8 +121,8 @@ export function promisifiedHandler<TEvent, TResult>(handler: Handler<TEvent, TRe
     if (asyncProm !== undefined && typeof asyncProm.then === "function") {
       // Mimics behaviour of lambda runtime, the first method of returning a result always wins.
       promise = Promise.race([callbackProm, asyncProm]);
-    } else if (asyncProm === undefined) {
-      // Handler returned undefined (also happens when missing return statement), resolve immediately
+    } else if (asyncProm === undefined && handler.length < 3) {
+      // Handler returned undefined and doesn't take a callback parameter, resolve immediately
       promise = Promise.resolve(undefined);
     }
     return promise;

From dcf36a2fa294bc314ef2bc9e1b59fdb0f6a699c3 Mon Sep 17 00:00:00 2001
From: Nicholas Hulston <nicholas.hulston@datadoghq.com>
Date: Thu, 12 Jun 2025 11:52:49 -0400
Subject: [PATCH 3/3] add unit test

---
 src/utils/handler.spec.ts | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/utils/handler.spec.ts b/src/utils/handler.spec.ts
index 271c85c39..e1100509f 100644
--- a/src/utils/handler.spec.ts
+++ b/src/utils/handler.spec.ts
@@ -296,4 +296,33 @@ describe("wrap", () => {
 
     expect(calledOriginalHandler).toBeFalsy();
   });
+
+  it("completes when handler returns undefined", async () => {
+    const handler: Handler = (event, context) => {
+      // No return statement, implicitly returns undefined
+    };
+
+    let calledStart = false;
+    let calledComplete = false;
+    let calledOriginalHandler = false;
+
+    const wrappedHandler = wrap(
+      handler,
+      async () => {
+        calledStart = true;
+      },
+      async () => {
+        calledComplete = true;
+      },
+    );
+
+    const result = await wrappedHandler({}, mockContext, () => {
+      calledOriginalHandler = true;
+    });
+
+    expect(result).toEqual(undefined);
+    expect(calledStart).toBeTruthy();
+    expect(calledComplete).toBeTruthy();
+    expect(calledOriginalHandler).toBeFalsy();
+  });
 });