Serialize falsy values for Durable Functions

src/WorkerChannel.ts

@@ -227,11 +227,18 @@ export class WorkerChannel implements IWorkerChannel {
       // explicitly set outputData to empty array to concat later
       response.outputData = [];
 
+      // As legacy behavior, falsy values get serialized to `null` in AzFunctions.
+      // This breaks Durable Functions expectations, where customers expect any
+      // JSON-serializable values to be preserved by the framework,
+      // so we check if we're serializing for durable and, if so, ensure falsy
+      // values get serialized.
+      let isDurableBinding = info?.bindings?.name?.type == 'activityTrigger';
+
       try {
-        if (result) {
+        if (result || (isDurableBinding && result != null)) {
           let returnBinding = info.getReturnBinding();
           // Set results from return / context.done
-          if (result.return) {
+          if (result.return || (isDurableBinding && result.return != null)) {
             if (this._v1WorkerBehavior) {
               response.returnValue = toTypedData(result.return);
             } else {

test/WorkerChannelTests.ts

@@ -86,6 +86,11 @@ describe('WorkerChannel', () => {
     direction: 1,
     dataType: 1
   };
+  const activityTriggerBinding = {
+    type: "activityTrigger",
+    direction: 1,
+    dataType: 1
+  };
   const httpInputBinding = { 
     type: "httpTrigger",
     direction: 0,
@@ -126,6 +131,11 @@ describe('WorkerChannel', () => {
       test: orchestrationTriggerBinding
     }
   };
+  const activityBinding = {
+    bindings: {
+      name: activityTriggerBinding
+    }
+  }
   const queueTriggerBinding = {
     bindings: {
       test: {
@@ -690,5 +700,46 @@ describe('WorkerChannel', () => {
       });
     });
 
+    it ('returns and serializes falsy value in Durable: ""', () => {
+      loader.getFunc.returns((context) => context.done(null, ""));
+      loader.getInfo.returns(new FunctionInfo(activityBinding));
+
+      sendInvokeMessage([], getHttpTriggerDataMock());
+
+      const expectedOutput = [];
+      const expectedReturnValue = { 
+        string: ""
+      };
+      assertInvocationSuccess(expectedOutput, expectedReturnValue);
+    });
+
+    it ('returns and serializes falsy value in Durable: 0', () => {
+      loader.getFunc.returns((context) => context.done(null, 0));
+      loader.getInfo.returns(new FunctionInfo(activityBinding));
+
+      sendInvokeMessage([], getHttpTriggerDataMock());
+
+      const expectedOutput = [];
+      const expectedReturnValue = { 
+        int: 0
+      };
+      assertInvocationSuccess(expectedOutput, expectedReturnValue);
+    });
+
+    it ('returns and serializes falsy value in Durable: false', () => {
+      loader.getFunc.returns((context) => context.done(null, false));
+      loader.getInfo.returns(new FunctionInfo(activityBinding));
+
+      sendInvokeMessage([], getHttpTriggerDataMock());
+
+      const expectedOutput = [];
+      const expectedReturnValue = { 
+        json: "false"
+      };
+      assertInvocationSuccess(expectedOutput, expectedReturnValue)
+    });
+
   });
+
+
 })