Add basic traverse function with tests

test/traverse-basic.spec.tsx

@@ -0,0 +1,39 @@
+import * as React from "react";
+// import ReactDOM from "react-dom";
+import { mount } from "enzyme";
+import CDepth2 from "./sample-components/depth-2-simple";
+import { traverse, FiberNode } from "../src";
+
+describe("Basic traverse test", () => {
+  let container: HTMLDivElement;
+
+  beforeEach(() => {
+    container = document.body.appendChild(document.createElement("div"));
+  });
+
+  afterEach(() => {
+    document.body.removeChild(container);
+  });
+
+  it("should render correctly", () => {
+    const rootRef = React.createRef<CDepth2>();
+
+    // ReactDOM.render(
+    //   <C text="text" ref={rootRef} />,
+    //   document.getElementById("container")
+    // );
+    mount(<CDepth2 ref={rootRef} />, { attachTo: container });
+
+    const mockCallback = jest.fn();
+
+    if (rootRef.current !== null) {
+      traverse(
+        (rootRef.current as any)._reactInternalFiber.child as FiberNode,
+        mockCallback
+      );
+    }
+
+    // The mock function is called twice
+    expect(mockCallback.mock.calls.length).toBe(2);
+  });
+});