From e9928ee53e6bd74af37e90acc57087bbdac412ba Mon Sep 17 00:00:00 2001
From: Matt McCutchen <matt@mattmccutchen.net>
Date: Tue, 7 Aug 2018 17:24:10 -0400
Subject: [PATCH] codemirror: EditorChangeCancellable.update is undefined for
 changes coming from undo/redo.

---
 types/codemirror/index.d.ts    | 5 +++--
 types/codemirror/test/index.ts | 9 +++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/types/codemirror/index.d.ts b/types/codemirror/index.d.ts
index 6dbfef6e60..f71f741659 100644
--- a/types/codemirror/index.d.ts
+++ b/types/codemirror/index.d.ts
@@ -716,8 +716,9 @@ declare namespace CodeMirror {
     }
 
     interface EditorChangeCancellable extends CodeMirror.EditorChange {
-        /** may be used to modify the change. All three arguments to update are optional, and can be left off to leave the existing value for that field intact. */
-        update(from?: CodeMirror.Position, to?: CodeMirror.Position, text?: string[]): void;
+        /** may be used to modify the change. All three arguments to update are optional, and can be left off to leave the existing value for that field intact.
+        If the change came from undo/redo, `update` is undefined and the change cannot be modified. */
+        update?(from?: CodeMirror.Position, to?: CodeMirror.Position, text?: string[]): void;
 
         cancel(): void;
     }
diff --git a/types/codemirror/test/index.ts b/types/codemirror/test/index.ts
index 9b9231b25c..431a20efb6 100644
--- a/types/codemirror/test/index.ts
+++ b/types/codemirror/test/index.ts
@@ -67,6 +67,15 @@ myCodeMirror.on(
   (instance: CodeMirror.Editor, line: CodeMirror.LineHandle, element: HTMLElement) => { }
 );
 
+myCodeMirror.on(
+  "beforeChange",
+  (instance: CodeMirror.Editor, change: CodeMirror.EditorChangeCancellable) => {
+    // $ExpectError
+    change.update();
+    if (change.update != null) change.update();
+  }
+);
+
 CodeMirror.registerHelper("lint", "javascript", {});
 
 myCodeMirror.isReadOnly();