Add simple unit tests for reducer.js

util/__tests__/__snapshots__/reducer.js.snap

@@ -0,0 +1,14 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`reducer commonSetState sets state from payload 1`] = `
+Immutable.Map {
+  "foo": "bar2",
+}
+`;
+
+exports[`reducer commonSetState sets state from payload 2`] = `
+Immutable.Map {
+  "foo": "bar",
+  "not-exist-key": "new-value",
+}
+`;

util/__tests__/reducer.js

@@ -1,5 +1,25 @@
+import { fromJS } from 'immutable';
+import { commonSetState } from '../reducer';
+
 describe('reducer', () => {
   describe('commonSetState', () => {
-    it('sets state from payload');
+    const initState = fromJS({
+      state: {
+        foo: 'bar',
+      },
+    });
+    it('sets state from payload', () => {
+      expect(
+        commonSetState(initState, {
+          payload: { key: 'foo', value: 'bar2' },
+        }).get('state')
+      ).toMatchSnapshot();
+
+      expect(
+        commonSetState(initState, {
+          payload: { key: 'not-exist-key', value: 'new-value' },
+        }).get('state')
+      ).toMatchSnapshot();
+    });
   });
 });