From 79f6df1f36bbb6f738dbcde3a961b79de0d54582 Mon Sep 17 00:00:00 2001
From: Han Kim <han@segment.com>
Date: Wed, 6 Jul 2016 13:45:20 -0700
Subject: [PATCH] push userId to data layer for track calls

---
 lib/index.js       | 3 +++
 test/index.test.js | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/lib/index.js b/lib/index.js
index fb6daf9..4059b24 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -83,6 +83,9 @@ GTM.prototype.page = function(page) {
 
 GTM.prototype.track = function(track) {
   var props = track.properties();
+  var id = this.analytics.user().id();
+  if (id) props.userId = id;
   props.event = track.event();
+
   push(props);
 };
diff --git a/test/index.test.js b/test/index.test.js
index 81aba73..b55e34e 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -67,6 +67,12 @@ describe('Google Tag Manager', function() {
         analytics.called(window.dataLayer.push, { event: 'some-event' });
       });
 
+      it('should send userId if it exists', function() {
+        analytics.user().id('pablo');
+        analytics.track('some-event');
+        analytics.called(window.dataLayer.push, { userId: 'pablo', event: 'some-event' });
+      });
+
       it('should send event with properties', function() {
         analytics.track('event', { prop: true });
         analytics.called(window.dataLayer.push, { event: 'event', prop: true });