narrow: Refactor logic for getting narrow title for browser/tab.

Creates a new function, `compute_narrow_title`, that returns a
string for the narrow title based on the current filter, so that
`update_narrow_title` can call that function, and then use the
returned string to set the narrow title.

Adds a node test for the new `compute_narrow_title` function for
cases that differ from the title generated by `filter.get_title`.

frontend_tests/node_tests/narrow.js

@@ -4,6 +4,7 @@ const {strict: assert} = require("assert");
 
 const {mock_esm, zrequire} = require("../zjsunit/namespace");
 const {run_test} = require("../zjsunit/test");
+const blueslip = require("../zjsunit/zblueslip");
 const $ = require("../zjsunit/zjquery");
 const {page_params} = require("../zjsunit/zpage_params");
 
@@ -813,3 +814,53 @@ run_test("narrow_to_compose_target PMs", ({override, override_rewire}) => {
     assert.equal(args.called, true);
     assert.deepEqual(args.operators, [{operator: "is", operand: "private"}]);
 });
+
+run_test("narrow_compute_title", () => {
+    // Only tests cases where the narrow title is different from the filter title.
+    let filter;
+
+    // Search & uncommon narrows
+    filter = new Filter([{operator: "search", operand: "potato"}]);
+    assert.equal(narrow.compute_narrow_title(filter), "translated: Search results");
+
+    filter = new Filter([{operator: "sender", operand: "me"}]);
+    assert.equal(narrow.compute_narrow_title(filter), "translated: Search results");
+
+    // Stream narrows
+    const sub = {
+        name: "Foo",
+        stream_id: 43,
+    };
+    stream_data.add_sub(sub);
+
+    filter = new Filter([
+        {operator: "stream", operand: "foo"},
+        {operator: "topic", operand: "bar"},
+    ]);
+    assert.equal(narrow.compute_narrow_title(filter), "#Foo > bar");
+
+    filter = new Filter([{operator: "stream", operand: "foo"}]);
+    assert.equal(narrow.compute_narrow_title(filter), "#Foo");
+
+    filter = new Filter([{operator: "stream", operand: "Elephant"}]);
+    assert.equal(narrow.compute_narrow_title(filter), "translated: Unknown stream #Elephant");
+
+    // Private messages with narrows
+    const joe = {
+        email: "joe@example.com",
+        user_id: 31,
+        full_name: "joe",
+    };
+    people.add_active_user(joe);
+
+    filter = new Filter([{operator: "pm-with", operand: "joe@example.com"}]);
+    assert.equal(narrow.compute_narrow_title(filter), "joe");
+
+    filter = new Filter([{operator: "pm-with", operand: "joe@example.com,sally@doesnotexist.com"}]);
+    blueslip.expect("warn", "Unknown emails: joe@example.com,sally@doesnotexist.com");
+    assert.equal(narrow.compute_narrow_title(filter), "translated: Invalid users");
+
+    filter = new Filter([{operator: "pm-with", operand: "sally@doesnotexist.com"}]);
+    blueslip.expect("warn", "Unknown emails: sally@doesnotexist.com");
+    assert.equal(narrow.compute_narrow_title(filter), "translated: Invalid user");
+});

static/js/narrow.js

@@ -124,42 +124,47 @@ export function set_narrow_title(title) {
     notifications.redraw_title();
 }
 
-function update_narrow_title(filter) {
+export function compute_narrow_title(filter) {
     const filter_title = filter.get_title();
-    const search_default = $t({defaultMessage: "Search results"});
 
-    if (filter_title !== undefined) {
-        if (filter.has_operator("stream")) {
-            if (!filter._sub) {
-                // The stream is not set because it does not currently
-                // exist (possibly due to a stream name change), or it
-                // is a private stream and the user is not subscribed.
-                set_narrow_title(filter_title);
-            } else if (filter.has_operator("topic")) {
-                const topic_name = filter.operands("topic")[0];
-                set_narrow_title("#" + filter_title + " > " + topic_name);
-            } else {
-                set_narrow_title("#" + filter_title);
-            }
-        } else if (filter.has_operator("pm-with")) {
-            const emails = filter.operands("pm-with")[0];
-            const user_ids = people.emails_strings_to_user_ids_string(emails);
-            if (user_ids !== undefined) {
-                const names = people.get_recipients(user_ids);
-                set_narrow_title(names);
-            } else {
-                if (emails.includes(",")) {
-                    set_narrow_title("Invalid users");
-                } else {
-                    set_narrow_title("Invalid user");
-                }
-            }
-        } else {
-            set_narrow_title(filter_title);
-        }
-    } else {
-        set_narrow_title(search_default);
+    if (filter_title === undefined) {
+        // Default result for uncommon narrow/search views.
+        return $t({defaultMessage: "Search results"});
     }
+
+    if (filter.has_operator("stream")) {
+        if (!filter._sub) {
+            // The stream is not set because it does not currently
+            // exist (possibly due to a stream name change), or it
+            // is a private stream and the user is not subscribed.
+            return filter_title;
+        }
+        if (filter.has_operator("topic")) {
+            const topic_name = filter.operands("topic")[0];
+            return "#" + filter_title + " > " + topic_name;
+        }
+        return "#" + filter_title;
+    }
+
+    if (filter.has_operator("pm-with")) {
+        const emails = filter.operands("pm-with")[0];
+        const user_ids = people.emails_strings_to_user_ids_string(emails);
+
+        if (user_ids !== undefined) {
+            return people.get_recipients(user_ids);
+        }
+        if (emails.includes(",")) {
+            return $t({defaultMessage: "Invalid users"});
+        }
+        return $t({defaultMessage: "Invalid user"});
+    }
+
+    return filter_title;
+}
+
+function update_narrow_title(filter) {
+    const narrow_title_string = compute_narrow_title(filter);
+    set_narrow_title(narrow_title_string);
 }
 
 export function reset_ui_state() {