typing_status: Switch sentinel "recipient" value to `null`.

This feels a bit more semantically appropriate: it more clearly says
"here's some information: there is no (relevant) recipient", rather
than "no information available".  (Both `null` and `undefined` in JS
can have either meaning, but `undefined` especially commonly means
the latter.)

Concretely, it ensures a bit more explicitness where the value
originates: a bare `return;` becomes `return null;`, reflecting the
fact that it is returning a quite informative value.

Also make the implementation more explicit about what's expected here,
replacing truthiness tests with `!== null`.  (A bit more idiomatic
would be `!= null`, which is equivalent when the value is well-typed
and a bit more robust to ill-typing bugs.  But lint complains about
that version.)

frontend_tests/node_tests/typing_status.js

@@ -16,7 +16,7 @@ run_test('basics', () => {
 
     // invalid conversation basically does nothing
     var worker = {};
-    typing_status.update(worker, undefined);
+    typing_status.update(worker, null);
 
     // Start setting up more testing state.
     typing_status.initialize_state();
@@ -128,7 +128,7 @@ run_test('basics', () => {
     });
 
     // Call stop with nothing going on.
-    call_handler(undefined);
+    call_handler(null);
     assert.deepEqual(typing_status.state, {
         next_send_start_time: undefined,
         idle_timer: undefined,
@@ -158,7 +158,7 @@ run_test('basics', () => {
     assert(events.idle_callback);
 
     // Explicitly stop alice.
-    call_handler(undefined);
+    call_handler(null);
     assert.deepEqual(typing_status.state, {
         next_send_start_time: undefined,
         idle_timer: undefined,
@@ -188,7 +188,7 @@ run_test('basics', () => {
     assert(events.idle_callback);
 
     // Switch to an invalid conversation.
-    call_handler(undefined);
+    call_handler(null);
     assert.deepEqual(typing_status.state, {
         next_send_start_time: undefined,
         idle_timer: undefined,
@@ -202,7 +202,7 @@ run_test('basics', () => {
     });
 
     // Switch to another invalid conversation.
-    call_handler(undefined);
+    call_handler(null);
     assert.deepEqual(typing_status.state, {
         next_send_start_time: undefined,
         idle_timer: undefined,

static/js/typing.js

@@ -26,7 +26,7 @@ function send_typing_notification_ajax(user_ids_array, operation) {
 function get_user_ids_array() {
     var user_ids_string = compose_pm_pill.get_user_ids_string();
     if (user_ids_string === "") {
-        return;
+        return null;
     }
 
     return people.user_ids_string_to_ids_array(user_ids_string);
@@ -65,14 +65,14 @@ exports.initialize = function () {
         // If our previous state was no typing notification, send a
         // start-typing notice immediately.
         var new_recipient =
-          is_valid_conversation() ? exports.get_recipient() : undefined;
+          is_valid_conversation() ? exports.get_recipient() : null;
         typing_status.update(worker, new_recipient);
     });
 
     // We send a stop-typing notification immediately when compose is
     // closed/cancelled
     $(document).on('compose_canceled.zulip compose_finished.zulip', function () {
-        typing_status.update(worker, undefined);
+        typing_status.update(worker, null);
     });
 };
 

static/shared/js/typing_status.js

@@ -17,7 +17,7 @@ export const state = {};
 
 /** Exported only for tests. */
 export function initialize_state() {
-    state.current_recipient =  undefined;
+    state.current_recipient = null;
     state.next_send_start_time = undefined;
     state.idle_timer = undefined;
 }
@@ -81,9 +81,9 @@ export function maybe_ping_server(worker, recipient) {
  * composing a stream message should be treated like composing no message at
  * all.
  *
- * Call with `new_recipient` of `undefined` when the user actively stops
+ * Call with `new_recipient` of `null` when the user actively stops
  * composing a message.  If the user switches from one set of recipients to
- * another, there's no need to call with `undefined` in between; the
+ * another, there's no need to call with `null` in between; the
  * implementation tracks the change and behaves appropriately.
  *
  * See docs/subsystems/typing-indicators.md for detailed background on the
@@ -92,12 +92,12 @@ export function maybe_ping_server(worker, recipient) {
  * @param {*} worker Callbacks for reaching the real world.  See typing.js
  *   for implementations.
  * @param {*} new_recipient The users the PM being composed is addressed to,
- *   as a sorted array of user IDs; or `undefined` if no PM is being
- *   composed anymore.
+ *   as a sorted array of user IDs; or `null` if no PM is being composed
+ *   anymore.
  */
 export function update(worker, new_recipient) {
     var current_recipient = state.current_recipient;
-    if (current_recipient) {
+    if (current_recipient !== null) {
         // We need to use _.isEqual for comparisons; === doesn't work
         // on arrays.
         if (_.isEqual(new_recipient, current_recipient)) {
@@ -117,7 +117,7 @@ export function update(worker, new_recipient) {
         stop_last_notification(worker);
     }
 
-    if (!new_recipient) {
+    if (new_recipient === null) {
         // If we are not talking to somebody we care about,
         // then there is no more action to take.
         return;

static/shared/js/typing_status.js.flow

@@ -14,5 +14,5 @@ type Worker = {|
 
 declare export function update(
   worker: Worker,
-  new_recipient: RecipientUserIds | void,
+  new_recipient: RecipientUserIds | null
 ): void;