Type selection object in editOnInput.

Summary:
Recently I merged D20582792— it was a quick fix to a a huge problem that was causing thousands of exceptions for our users.

It could've been prevented with better typing, but wasn't caught by flow because the type of the selection object was assumed to be null.

Here I implement the type of the selection object, and in the process catch other elements that could be null and add safeguards to make our code more reliable. This are places where the previous implementation would've thrown an exception.

Reviewed By: kedromelon

Differential Revision: D20636236

fbshipit-source-id: 6e27040120256e3885bfdd2d9cbedf5e57988062

Author: mrkev

src/component/handlers/edit/editOnInput.js

@@ -11,6 +11,7 @@
 
 'use strict';
 
+import type {SelectionObject} from 'DraftDOMTypes';
 import type DraftEditor from 'DraftEditor.react';
 
 const DraftModifier = require('DraftModifier');
@@ -65,14 +66,14 @@ function editOnInput(editor: DraftEditor, e: SyntheticInputEvent<>): void {
   }
   // at this point editor is not null for sure (after input)
   const castedEditorElement: HTMLElement = (editor.editor: any);
-  const domSelection = castedEditorElement.ownerDocument.defaultView.getSelection();
+  const domSelection: SelectionObject = castedEditorElement.ownerDocument.defaultView.getSelection();
 
   const {anchorNode, isCollapsed} = domSelection;
   const isNotTextOrElementNode =
     anchorNode?.nodeType !== Node.TEXT_NODE &&
     anchorNode?.nodeType !== Node.ELEMENT_NODE;
 
-  if (isNotTextOrElementNode) {
+  if (anchorNode == null || isNotTextOrElementNode) {
     // TODO: (t16149272) figure out context for this change
     return;
   }
@@ -86,10 +87,14 @@ function editOnInput(editor: DraftEditor, e: SyntheticInputEvent<>): void {
     // https://chromium.googlesource.com/chromium/src/+/a3b600981286b135632371477f902214c55a1724
     // To work around, we'll merge the sibling text nodes back into this one.
     const span = anchorNode.parentNode;
+    if (span == null) {
+      // Handle null-parent case.
+      return;
+    }
     anchorNode.nodeValue = span.textContent;
     for (
       let child = span.firstChild;
-      child !== null;
+      child != null;
       child = child.nextSibling
     ) {
       if (child !== anchorNode) {

src/component/utils/DraftDOMTypes.js

@@ -0,0 +1,28 @@
+/**
+ * (c) Facebook, Inc. and its affiliates. Confidential and proprietary.
+ *
+ * Types for things in the DOM used in Draft.js. These should eventaully be
+ * added to the flow DOM lib itself.
+ *
+ * @emails oncall+draft_js
+ * @flow strict
+ * @format
+ */
+
+'use strict';
+
+// https://developer.mozilla.org/en-US/docs/Web/API/Selection
+export type SelectionObject = {|
+  /**
+   * Returns the Node in which the selection begins. Can return null if
+   * selection never existed in the document (e.g., an iframe that was
+   * never clicked on). */
+  anchorNode: ?Node,
+  anchorOffset: number,
+  focusNode: ?Node,
+  focusOffset: number,
+  isCollapsed: boolean,
+  rangeCount: number,
+  type: string,
+  // ...etc. This is a non-exhaustive definition.
+|};