fixed unit test usage

Author: hipertracker

src/__tests__/sum-test.js

@@ -1,6 +1,9 @@
 jest.dontMock('../sum'); // or jest.autoMockOff();
 
-import sum from '../sum'
+// cannot use es6 modules because
+// jest.dontMock & jest,autoMockOff()
+// do not understand ES6 modules
+const sum = require('../sum');
 
 describe('sum', function () {
     it('adds 1 + 2 to equal 3', function () {

src/sum.js

@@ -1,3 +1,5 @@
 // ES6 code
 var sum = (a, b) => a + b;
-export default sum;
+
+export default sum
+