Fix double callback invoke on unhandled exception

Author: adamjmcgrath

src/types.ts

@@ -256,7 +256,7 @@ export function createOptionalCallbackFunction<T, A extends unknown[]>(
     if (isErrorFirstCallback(possibleCallback)) {
       try {
         const result = syncVersion(...(args.slice(0, -1) as A));
-        possibleCallback(null, result);
+        process.nextTick(() => possibleCallback(null, result));
       } catch (err) {
         possibleCallback(err instanceof Error ? err : new Error("Unknown error"));
       }

test/types-tests.spec.ts

@@ -0,0 +1,52 @@
+/// <reference lib="dom" />
+import * as types from "../src/types";
+import { expect } from "chai";
+
+describe("createOptionalCallbackFunction", function () {
+  it("should not execute callback twice when callback throws unhandled exception", function (done) {
+    const syncFn = (a: number, b: number) => a + b;
+    const flexibleFn = types.createOptionalCallbackFunction(syncFn);
+
+    let callbackExecutionCount = 0;
+
+    // Store and remove existing unhandled exception listeners
+    const existingListeners = process.rawListeners("uncaughtException");
+    process.removeAllListeners("uncaughtException");
+
+    process.once("uncaughtException", (err) => {
+      // Restore unhandled exception listeners
+      existingListeners.forEach((listener) => {
+        process.on("uncaughtException", listener as NodeJS.UncaughtExceptionListener);
+      });
+
+      expect(err.message).to.equal("Callback threw an error");
+      expect(callbackExecutionCount).to.equal(1);
+      done();
+    });
+
+    flexibleFn(2, 3, (err, result) => {
+      callbackExecutionCount++;
+      expect(err).to.be.null;
+      expect(result).to.equal(5);
+
+      throw new Error("Callback threw an error");
+    });
+  });
+
+  it("should defer callback execution in success case", function (done) {
+    const syncFn = (a: number, b: number) => a + b;
+    const flexibleFn = types.createOptionalCallbackFunction(syncFn);
+
+    let callbackExecuted = false;
+
+    flexibleFn(2, 3, (err, result) => {
+      callbackExecuted = true;
+      expect(err).to.be.null;
+      expect(result).to.equal(5);
+      done();
+    });
+
+    // Callback should be asynchronously deferred
+    expect(callbackExecuted).to.be.false;
+  });
+});