Add feature to use uid only without cid

README.md

@@ -71,6 +71,11 @@ If you want to set User Id you can add it into options:
 ```javascript
 var visitor = ua('UA-XXXX-XX', {uid: 'as8eknlll'});
 ```
+If you want to only use `uid` to identify user and not using the clientID, just include the options `identifyByUserId: true`. This is useful for those who mainly track user base on UserId.
+```javascript
+var visitor = ua('UA-XXXX-XX', {uid: 'as8eknlll', identifyByUserId: true});
+```
+
 [see about User Id](https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters#uid)
 
 

lib/index.js

@@ -54,7 +54,9 @@ var Visitor = module.exports.Visitor = function (tid, cid, options, context, per
   this._persistentParams = persistentParams || {};
 
   this.tid = tid || this.options.tid;
-  this.cid = this._determineCid(cid, this.options.cid, (this.options.strictCidFormat !== false));
+  if(!this.options.identifyByUserId || (this.options.identifyByUserId && !this.options.uid)) {
+    this.cid = this._determineCid(cid, this.options.cid, (this.options.strictCidFormat !== false));
+  }
   if(this.options.uid) {
     this.uid = this.options.uid;
   }

test/index.js

@@ -114,6 +114,41 @@ describe("ua", function () {
 		visitor.cid.should.equal(options.cid)
 	});
 
+	context("identifyByUserId", function() {
+		var uuidV4Spy
+		before(function() {
+			uuidV4Spy = sinon.spy(uuid, 'v4')
+		})
+		after(function() {
+			uuidV4Spy.restore()
+		})
+		afterEach(function() {
+			uuidV4Spy.reset()
+		})
+		it("should not genereate cid if uid exists and identifyByUserId is true", function() {
+			var options = {
+				tid: "UA-XXXXX-XX",
+				uid: "some-user-id",
+				identifyByUserId: true
+			};
+
+			var visitor = ua(options);
+			uuidV4Spy.calledOnce.should.equal(false);
+			visitor.should.not.have.property("cid")
+		});
+
+		it("should still generate new cid (UUID) if identifyByUserId is true but uid does not exist", function () {
+			var options = {
+				tid: "UA-XXXXX-XX",
+				identifyByUserId: true
+			};
+
+			var visitor = ua(options);
+			uuidV4Spy.calledOnce.should.equal(true, "uuid.v4 should've been called once");
+			utils.isUuid(visitor.cid).should.equal(true, "A valid random UUID should have been generated")
+		});
+	})
+
 
 	describe("params", function () {