filter: Cache/store sorted_term_types.

Given that the sorted term types depends only on the filter, it makes
sense to only build them once and cache the results.

frontend_tests/node_tests/filter.js

@@ -1179,10 +1179,31 @@ run_test('term_type', () => {
         {operator: 'sender', operand: 'steve@foo.com'},
         {operator: 'stream', operand: 'Verona'},
     ];
-    const filter = new Filter(terms);
+    let filter = new Filter(terms);
     const term_types = filter.sorted_term_types();
 
     assert.deepEqual(term_types, ['stream', 'topic', 'sender']);
+
+    // test caching of term types
+    // init and stub
+    filter = new Filter(terms);
+    filter.stub = filter._build_sorted_term_types;
+    filter._build_sorted_term_types = function () {
+        this._build_sorted_term_types_called = true;
+        return this.stub();
+    };
+
+    // uncached trial
+    filter._build_sorted_term_types_called = false;
+    const built_terms = filter.sorted_term_types();
+    assert.deepEqual(built_terms, ['stream', 'topic', 'sender']);
+    assert(filter._build_sorted_term_types_called);
+
+    // cached trial
+    filter._build_sorted_term_types_called = false;
+    const cached_terms = filter.sorted_term_types();
+    assert.deepEqual(cached_terms, ['stream', 'topic', 'sender']);
+    assert(!filter._build_sorted_term_types_called);
 });
 
 run_test('first_valid_id_from', () => {

static/js/filter.js

@@ -502,6 +502,13 @@ Filter.prototype = {
     },
 
     sorted_term_types: function () {
+        if (this._sorted_term_types === undefined) {
+            this._sorted_term_types = this._build_sorted_term_types();
+        }
+        return this._sorted_term_types;
+    },
+
+    _build_sorted_term_types: function () {
         const terms = this._operators;
         const term_types = terms.map(Filter.term_type);
         const sorted_terms = Filter.sorted_term_types(term_types);