Add selection start tests in regards to issue facebook#11806

packages/react-dom/src/client/__tests__/getSelectionStart-test.js

@@ -0,0 +1,45 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+const React = require('react');
+const ReactDOM = require('react-dom');
+
+describe('getSelectionText', () => {
+
+  describe('when the node is a text area', () => {
+    it('has a value for selectionStart', () => {
+      const container = document.createElement('div');
+      ReactDOM.render(<textarea />, container);
+
+      expect(container.firstChild.selectionStart).not.toBeUndefined();
+    });
+  });
+
+  describe('when the node is an input', () => {
+    it('has a value for selectionStart', () => {
+      const container = document.createElement('div');
+      ReactDOM.render(<input />, container);
+
+      expect(container.firstChild.selectionStart).not.toBeUndefined();
+    });
+  });
+
+  describe('when the node is a content editable div', () => {
+    it('has an undefined value for selectionStart', () => {
+      const container = document.createElement('div');
+      ReactDOM.render(<div />, container);
+      container.firstChild.contentEditable = true;
+
+      expect(container.firstChild).toBeTruthy();
+      expect(container.firstChild.selectionStart).toBeUndefined();
+    })
+  })
+});