wip eslint: Disable no-undef-init; undo its effects on our code.

.eslintrc.yaml

@@ -68,6 +68,18 @@ rules:
   no-plusplus: off
   no-nested-ternary: off
 
+  # We prefer `let foo = undefined;` over `let foo;`, not vice versa.
+  #
+  # Sadly there doesn't seem to be a rule to say what we want, but at least
+  # turn off the one that says the opposite.
+  #
+  # This rule is dubious to begin with, because explicit is better than
+  # implicit.  Then for us, the overriding reason we definitely don't
+  # want it is that plain `let foo;` can trigger a Flow bug:
+  #   https://github.com/facebook/flow/issues/8526
+  # while `let foo = undefined;` makes Flow handle the logic smoothly.
+  no-undef-init: off
+
   # Allow multiple classes per file.
   #
   # It's best to have just one class per file, but there are cases

src/caughtup/caughtUpReducer.js

@@ -90,7 +90,7 @@ export default (state: CaughtUpState = initialState, action: Action): CaughtUpSt
         return state;
       }
       const key = JSON.stringify(action.narrow);
-      let caughtUp;
+      let caughtUp = undefined;
       if (action.foundNewest !== undefined && action.foundOldest !== undefined) {
         /* This should always be the case for Zulip Server v1.8 or newer. */
         caughtUp = { older: action.foundOldest, newer: action.foundNewest };

src/compose/ComposeMenu.js

@@ -123,7 +123,7 @@ class ComposeMenu extends PureComponent<Props> {
     // from this library when in the test environment.
     const DocumentPicker = (await import('react-native-document-picker')).default;
 
-    let response;
+    let response = undefined;
     try {
       response = (await DocumentPicker.pick({
         type: [DocumentPicker.types.allFiles],

src/message/renderMessages.js

@@ -10,7 +10,6 @@ export default (
 ): RenderedSectionDescriptor[] => {
   const showHeader = !isPrivateOrGroupNarrow(narrow) && !isTopicNarrow(narrow);
 
-  // eslint-disable-next-line no-undef-init
   let prevItem = undefined;
   const sections = [{ key: 0, data: [], message: {} }];
   messages.forEach(item => {

src/webview/js/generatedEs3.js

@@ -545,7 +545,7 @@ var compiledWebviewJs = (function (exports) {
 
   var scrollEventsDisabled = true;
   var hasLongPressed = false;
-  var longPressTimeout;
+  var longPressTimeout = undefined;
   var lastTouchPositionX = -1;
   var lastTouchPositionY = -1;
 

src/webview/js/js.js

@@ -431,7 +431,7 @@ const sendScrollMessageIfListShort = () => {
 let scrollEventsDisabled = true;
 
 let hasLongPressed = false;
-let longPressTimeout;
+let longPressTimeout = undefined;
 let lastTouchPositionX = -1;
 let lastTouchPositionY = -1;