Fix Mixpanel distinct_id for anonymous events

packages/destination-actions/src/destinations/mixpanel/trackEvent/__tests__/index.test.ts

@@ -263,6 +263,36 @@ describe('Mixpanel.trackEvent', () => {
     }
   })
 
+  it('should allow an empty, but present, distinct_id', async () => {
+    const event = createTestEvent({ timestamp, event: 'Test Event' })
+
+    nock('https://api.mixpanel.com').post('/import?strict=1').reply(200, {})
+
+    const responses = await testDestination.testAction('trackEvent', {
+      event,
+      settings: {
+        projectToken: MIXPANEL_PROJECT_TOKEN,
+        apiSecret: MIXPANEL_API_SECRET
+      },
+      useDefaultMappings: true,
+      mapping: {
+        distinct_id: '' // Map an empty distinct_id for the action.
+      }
+    })
+    expect(responses.length).toBe(1)
+    expect(responses[0].status).toBe(200)
+    expect(responses[0].data).toMatchObject({})
+    expect(responses[0].options.json).toMatchObject([
+      {
+        event: 'Test Event',
+        properties: expect.objectContaining({
+          ...expectedProperties,
+          distinct_id: '' // Expect an empty string `distinct_id` returned.
+        })
+      }
+    ])
+  })
+
   it('should invoke performBatch for batches', async () => {
     const events = [
       createTestEvent({ timestamp, event: 'Test Event1' }),

packages/destination-actions/src/destinations/mixpanel/trackEvent/functions.ts

@@ -17,12 +17,24 @@ export function getEventProperties(payload: Payload, settings: Settings): Mixpan
     browserVersion = getBrowserVersion(payload.userAgent)
   }
   const integration = payload.context?.integration as Record<string, string>
+
+  // When the `distinct_id` property is undefined we add the property in as a blank string,
+  // so that we can support Mixpanel events that aren't tied to a specific user.
+  //
+  // The `distinct_id` property is, according to documentation, supposed to
+  // ALWAYS be present, either with an identifying value or with an empty
+  // string value to denote that the event should be excluded from behavioral
+  // analysis.
+  //
+  // @see https://developer.mixpanel.com/reference/import-events#propertiesdistinct_id
+  const distinct_id = payload.distinct_id ?? ''
+
   return {
     time: time,
     ip: payload.ip,
     id: payload.distinct_id,
     $anon_id: payload.anonymous_id,
-    distinct_id: payload.distinct_id,
+    distinct_id,
     $app_build_number: payload.app_build,
     $app_version_string: payload.app_version,
     $app_namespace: payload.app_namespace,