Add message_list.is_at_end() helper.

frontend_tests/node_tests/message_list.js

@@ -130,6 +130,16 @@ run_test('basics', () => {
 run_test('prev_next', () => {
     var list = new MessageList({});
 
+    assert.equal(list.prev(), undefined);
+    assert.equal(list.next(), undefined);
+    assert.equal(list.is_at_end(), false);
+
+    // try to confuse things with bogus selected id
+    list.data.set_selected_id(33);
+    assert.equal(list.prev(), undefined);
+    assert.equal(list.next(), undefined);
+    assert.equal(list.is_at_end(), false);
+
     var messages = [{id: 30}, {id: 40}, {id: 50}, {id: 60}];
     list.append(messages, true);
     assert.equal(list.prev(), undefined);
@@ -139,6 +149,7 @@ run_test('prev_next', () => {
     list.data.set_selected_id(45);
     assert.equal(list.prev(), undefined);
     assert.equal(list.next(), undefined);
+    assert.equal(list.is_at_end(), false);
 
     list.data.set_selected_id(30);
     assert.equal(list.prev(), undefined);
@@ -147,10 +158,12 @@ run_test('prev_next', () => {
     list.data.set_selected_id(50);
     assert.equal(list.prev(), 40);
     assert.equal(list.next(), 60);
+    assert.equal(list.is_at_end(), false);
 
     list.data.set_selected_id(60);
     assert.equal(list.prev(), 50);
     assert.equal(list.next(), undefined);
+    assert.equal(list.is_at_end(), true);
 });
 
 run_test('message_range', () => {

static/js/message_list.js

@@ -99,6 +99,10 @@ exports.MessageList.prototype = {
         return this.data.next();
     },
 
+    is_at_end: function () {
+        return this.data.is_at_end();
+    },
+
     nth_most_recent_id: function (n) {
         return this.data.nth_most_recent_id(n);
     },

static/js/message_list_data.js

@@ -78,6 +78,22 @@ MessageListData.prototype = {
         return this._items[i + 1].id;
     },
 
+    is_at_end: function () {
+        if (this._selected_id === -1) {
+            return false;
+        }
+
+        var n = this._items.length;
+
+        if (n === 0) {
+            return false;
+        }
+
+        var last_msg = this._items[n-1];
+
+        return (last_msg.id === this._selected_id);
+    },
+
     nth_most_recent_id: function (n) {
         var i = this._items.length - n;
         if (i < 0) {