Fix Issue/59 syntax error calling custom vendor api without params (#60)

* (refactor) fix error calling api with undefined params
* (refactor) validate arguments make it through vendor api
* (docs) document custom api method signature

Author: potench

docs/Guides.md

@@ -33,6 +33,12 @@ A name of the vendor service to be returned as part of tracking response if defi
 An object, a class instance or a function which returns an object which exposes tracking APIs. You don't have to define `pageView` or `track` api, but keep in mind that `react-metrics` will assume those methods to exist for auto page view and declarative tracking and throws when not available.
 You can define `name` property in your api, which will be returned as part of tracking response.
 
+Custom `api` methods can take 3 arguments:
+
+```
+someMethod(eventType?: string, eventData?: Object, shouldMergeWithDefaultObject?: boolean)
+```
+
 Example:
 
 ```javascript

src/core/createMetrics.js

@@ -165,6 +165,7 @@ export class Metrics extends EventEmitter {
      */
     _callServices(type, promise) {
         return promise.then(params => {
+            params = params || [];
             const results = [];
             const services = this.services;
             const requestTimeout = this.requestTimeout;

test/core/createMetrics.spec.js

@@ -223,6 +223,30 @@ describe("Metrics", () => {
         }, 0);
     });
 
+    it("allows a custom vendor method without arguments", done => {
+        const metricsInstance = new Metrics(metricsConfig);
+        metricsInstance.listen(event => {
+            expect(event)
+                .to.have.property("type")
+                .and.equal("someMethod");
+            expect(event)
+                .to.have.property("status")
+                .and.equal("success");
+            done();
+        });
+        metricsInstance.api.someMethod();
+    });
+
+    it("passes first 2 arguments through custom vendor api methods", done => {
+        const metricsInstance = new Metrics(metricsConfig);
+        const stub = sinon.stub(console, "log", (logName, event) => {
+            expect(logName).to.equal("argumentTest 1 2 undefined");
+            done();
+            stub.restore();
+        });
+        metricsInstance.api.argumentTest(1, 2, 3);
+    });
+
     it("should have console log when debug flag is set", done => {
         const missingPageDefaultsConfig = Object.assign({}, metricsConfig, {
             debug: true

test/metrics.config.js

@@ -31,6 +31,17 @@ export default {
                             user
                         });
                     });
+                },
+                someMethod() {
+                    return new Promise(resolve => {
+                        resolve();
+                    });
+                },
+                argumentTest(a, b, c) {
+                    console.log(`argumentTest ${a} ${b} ${c}`, b);
+                    return new Promise(resolve => {
+                        resolve();
+                    });
                 }
             }
         }