wip: code formatting nits continued

test/differenceBy.spec.js

@@ -0,0 +1,22 @@
+import { slice } from './utils';
+import differenceBy from '../src/differenceBy';
+
+describe('differenceBy', () => {
+    it('should accept an `iteratee`', () => {
+        let actual = differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor);
+        expect(actual).toEqual([1.2]);
+
+        actual = differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], 'x');
+        expect(actual).toEqual([{ x: 2 }]);
+    });
+
+    it('should provide correct `iteratee` arguments', () => {
+        let args;
+
+        differenceBy([2.1, 1.2], [2.3, 3.4], function () {
+            args || (args = slice.call(arguments));
+        });
+
+        expect(args).toEqual([2.3]);
+    });
+});