diff --git a/frontend_tests/node_tests/compose.js b/frontend_tests/node_tests/compose.js
index d7ba2e2819..238f803789 100644
--- a/frontend_tests/node_tests/compose.js
+++ b/frontend_tests/node_tests/compose.js
@@ -865,12 +865,12 @@ function test_raw_file_drop(raw_drop_func) {
 
 run_test('warn_if_private_stream_is_linked', () => {
     stream_data.add_sub(compose_state.stream_name(), {
-        subscribers: LazySet([1, 2]),
+        subscribers: new LazySet([1, 2]),
     });
 
     let denmark = {
         name: 'Denmark',
-        subscribers: LazySet([1, 2, 3]),
+        subscribers: new LazySet([1, 2, 3]),
     };
 
     function test_noop_case(invite_only) {
@@ -913,7 +913,7 @@ run_test('warn_if_private_stream_is_linked', () => {
     denmark = {
         invite_only: true,
         name: 'Denmark',
-        subscribers: LazySet([1]),
+        subscribers: new LazySet([1]),
     };
 
     compose.warn_if_private_stream_is_linked(denmark);
diff --git a/frontend_tests/node_tests/lazy_set.js b/frontend_tests/node_tests/lazy_set.js
index 74cddb7c9a..5b961be190 100644
--- a/frontend_tests/node_tests/lazy_set.js
+++ b/frontend_tests/node_tests/lazy_set.js
@@ -8,7 +8,7 @@ const LazySet = zrequire('lazy_set').LazySet;
 */
 
 run_test('map', () => {
-    const ls = LazySet([1, 2]);
+    const ls = new LazySet([1, 2]);
 
     const triple = (n) => n * 3;
 
@@ -17,7 +17,7 @@ run_test('map', () => {
 
 run_test('conversions', () => {
     blueslip.set_test_data('error', 'not a number');
-    const ls = LazySet([1, 2]);
+    const ls = new LazySet([1, 2]);
     ls.add('3');
     assert(ls.has('3'));
 });
diff --git a/frontend_tests/node_tests/typeahead_helper.js b/frontend_tests/node_tests/typeahead_helper.js
index 6aef0e2b53..ff202dd154 100644
--- a/frontend_tests/node_tests/typeahead_helper.js
+++ b/frontend_tests/node_tests/typeahead_helper.js
@@ -35,9 +35,9 @@ stream_data.create_streams([
 ]);
 
 run_test('sort_streams', () => {
-    const popular = LazySet([1, 2, 3, 4, 5, 6]);
+    const popular = new LazySet([1, 2, 3, 4, 5, 6]);
 
-    const unpopular = LazySet([1]);
+    const unpopular = new LazySet([1]);
 
     let test_streams = [
         {name: 'Dev', pin_to_top: false, subscribers: unpopular, subscribed: true},
diff --git a/static/js/lazy_set.js b/static/js/lazy_set.js
index c6ddce9d0c..5a22d1e059 100644
--- a/static/js/lazy_set.js
+++ b/static/js/lazy_set.js
@@ -1,4 +1,4 @@
-exports.LazySet = function (vals) {
+export class LazySet {
     /*
         This class is optimized for a very
         particular use case.
@@ -20,63 +20,62 @@ exports.LazySet = function (vals) {
         as has/add/delete, we convert it over
         to a set for a one-time cost.
     */
-    const self = {};
-    self.arr = vals;
-    self.set = undefined;
 
-    self.keys = function () {
-        if (self.set !== undefined) {
-            return Array.from(self.set);
-        }
-        return self.arr;
-    };
-
-    function make_set() {
-        if (self.set !== undefined) {
-            return;
-        }
-        self.set = new Set(self.arr);
-        self.arr = undefined;
+    constructor(vals) {
+        this.arr = vals;
+        this.set = undefined;
     }
 
-    self.num_items = function () {
-        if (self.set !== undefined) {
-            return self.set.size;
+    keys() {
+        if (this.set !== undefined) {
+            return Array.from(this.set);
+        }
+        return this.arr;
+    }
+
+    _make_set() {
+        if (this.set !== undefined) {
+            return;
+        }
+        this.set = new Set(this.arr);
+        this.arr = undefined;
+    }
+
+    num_items() {
+        if (this.set !== undefined) {
+            return this.set.size;
         }
 
-        return self.arr.length;
-    };
+        return this.arr.length;
+    }
 
-    self.map = function (f) {
-        return _.map(self.keys(), f);
-    };
+    map(f) {
+        return _.map(this.keys(), f);
+    }
 
-    self.has = function (v) {
-        make_set();
-        const val = self._clean(v);
-        return self.set.has(val);
-    };
+    has(v) {
+        this._make_set();
+        const val = this._clean(v);
+        return this.set.has(val);
+    }
 
-    self.add = function (v) {
-        make_set();
-        const val = self._clean(v);
-        self.set.add(val);
-    };
+    add(v) {
+        this._make_set();
+        const val = this._clean(v);
+        this.set.add(val);
+    }
 
-    self.delete = function (v) {
-        make_set();
-        const val = self._clean(v);
-        return self.set.delete(val);
-    };
+    delete(v) {
+        this._make_set();
+        const val = this._clean(v);
+        return this.set.delete(val);
+    }
 
-    self._clean = function (v) {
+    _clean(v) {
         if (typeof v !== 'number') {
             blueslip.error('not a number');
             return parseInt(v, 10);
         }
         return v;
-    };
-
-
-    return self;
-};
+    }
+}
diff --git a/static/js/stream_data.js b/static/js/stream_data.js
index ca5026e6ef..08b22f3dff 100644
--- a/static/js/stream_data.js
+++ b/static/js/stream_data.js
@@ -168,7 +168,7 @@ exports.unsubscribe_myself = function (sub) {
 
 exports.add_sub = function (stream_name, sub) {
     if (!_.has(sub, 'subscribers')) {
-        sub.subscribers = LazySet([]);
+        sub.subscribers = new LazySet([]);
     }
 
     stream_info.set(stream_name, sub);
@@ -603,7 +603,7 @@ exports.maybe_get_stream_name = function (stream_id) {
 };
 
 exports.set_subscribers = function (sub, user_ids) {
-    sub.subscribers = LazySet(user_ids || []);
+    sub.subscribers = new LazySet(user_ids || []);
 };
 
 exports.add_subscriber = function (stream_name, user_id) {