fix: ship handler.mjs as the sole Lambda entry point

.gitlab/scripts/publish_npm.sh

@@ -29,5 +29,4 @@ if [ -d "./dist" ]; then
     rm -rf ./dist
 fi
 yarn build
-cp ./dist/handler.cjs ./dist/handler.js
 npm publish

Dockerfile

@@ -13,6 +13,13 @@ RUN yarn install
 # Build the lambda layer
 RUN yarn build
 RUN cp -r dist /nodejs/node_modules/datadog-lambda-js
+# The CJS shim at dist/handler.js exists to route ESM Lambdas through
+# handler.mjs when the npm package is used directly (the publish scripts no
+# longer copy handler.cjs to handler.js). In the layer, Lambda's bootstrap
+# resolves `handler.handler` to handler.mjs as long as handler.js is absent,
+# which handles both CJS and ESM user code via the async load(). Drop the
+# shim here so the layer keeps that direct-to-mjs behavior.
+RUN rm /nodejs/node_modules/datadog-lambda-js/handler.js
 RUN cp ./src/runtime/module_importer.js /nodejs/node_modules/datadog-lambda-js/runtime
 RUN node <<'EOF'
 const fs = require("fs");

scripts/publish_prod.sh

@@ -95,7 +95,6 @@ else
         rm -rf ./dist
     fi
     yarn build
-    cp ./dist/handler.cjs ./dist/handler.js
     yarn publish --new-version "$NEW_VERSION"
 fi
 

src/handler.js

@@ -0,0 +1,63 @@
+// CJS entry shim used by AWS Lambda's bootstrap when the handler string
+// resolves to `node_modules/datadog-lambda-js/dist/handler.handler`.
+//
+// Lambda's resolver searches for `.js` before `.mjs`, so this file is what
+// gets loaded for both CJS and ESM user functions. We branch at runtime:
+//
+//   - CJS user code: synchronously delegate to handler.cjs (preserves the
+//     prior fast-path behavior — no extra dynamic import on cold start).
+//
+//   - ESM user code: expose an async handler that lazily `import()`s
+//     handler.mjs on the first invocation. handler.mjs's async `load()`
+//     transparently handles both CJS and ESM user modules, so this also
+//     fixes the ERR_REQUIRE_ESM failure mode reported in
+//     https://github.com/DataDog/datadog-lambda-js/issues/782.
+
+"use strict";
+
+const fs = require("fs");
+const path = require("path");
+
+function userIsEsm() {
+  const handlerEnv = process.env.DD_LAMBDA_HANDLER || process.env._HANDLER || "";
+  if (/\.mjs(\..*)?$/.test(handlerEnv)) {
+    return true;
+  }
+
+  const taskRoot = process.env.LAMBDA_TASK_ROOT || process.cwd();
+  try {
+    const pkgPath = path.join(taskRoot, "package.json");
+    const raw = fs.readFileSync(pkgPath, "utf8");
+    const pkg = JSON.parse(raw);
+    if (pkg && pkg.type === "module") {
+      return true;
+    }
+  } catch (_) {
+    // package.json missing or unreadable — assume CJS, matches prior behavior.
+  }
+
+  return false;
+}
+
+if (userIsEsm()) {
+  let cachedHandler;
+  let cachedError;
+
+  exports.handler = async function ddEsmHandler(event, context) {
+    if (cachedError) {
+      throw cachedError;
+    }
+    if (!cachedHandler) {
+      try {
+        const mod = await import("./handler.mjs");
+        cachedHandler = mod.handler;
+      } catch (error) {
+        cachedError = error;
+        throw error;
+      }
+    }
+    return cachedHandler(event, context);
+  };
+} else {
+  module.exports = require("./handler.cjs");
+}