hashchange: Add hash_util.get_hash_category().

This replaces hashchange.get_main_hash(), which had
a slightly misleading name.  Also, moving this to
hash_util forces us to keep 100% coverage on it.

frontend_tests/node_tests/hash_util.js

@@ -63,6 +63,25 @@ run_test('hash_util', () => {
     encode_decode_operand(operator, operand, 'testing.20123');
 });
 
+run_test('test_get_hash_category', () => {
+    assert.deepEqual(
+        hash_util.get_hash_category('streams/subscribed'),
+        'streams'
+    );
+    assert.deepEqual(
+        hash_util.get_hash_category('#settings/display-settings'),
+        'settings'
+    );
+    assert.deepEqual(
+        hash_util.get_hash_category('#drafts'),
+        'drafts'
+    );
+    assert.deepEqual(
+        hash_util.get_hash_category('invites'),
+        'invites'
+    );
+});
+
 run_test('test_parse_narrow', () => {
     assert.deepEqual(
         hash_util.parse_narrow(['narrow', 'stream', '11-social']),

static/js/hash_util.js

@@ -2,6 +2,11 @@ var hash_util = (function () {
 
 var exports = {};
 
+exports.get_hash_category = function (hash) {
+    // given "#streams/subscribed", returns "streams"
+    return hash ? hash.replace(/^#/, "").split(/\//)[0] : "";
+};
+
 // Some browsers zealously URI-decode the contents of
 // window.location.hash.  So we hide our URI-encoding
 // by replacing % with . (like MediaWiki).

static/js/hashchange.js

@@ -59,10 +59,6 @@ var state = {
     old_hash: typeof window !== "undefined" ? window.location.hash : "#",
 };
 
-function get_main_hash(hash) {
-    return hash ? hash.replace(/^#/, "").split(/\//)[0] : "";
-}
-
 function get_hash_components() {
     var hash = window.location.hash.split(/\//);
 
@@ -75,7 +71,7 @@ function get_hash_components() {
 function is_overlay_hash(hash) {
     // Hash changes within this list are overlays and should not unnarrow (etc.)
     var overlay_list = ["streams", "drafts", "settings", "organization", "invite"];
-    var main_hash = get_main_hash(hash);
+    var main_hash = hash_util.get_hash_category(hash);
 
     return overlay_list.indexOf(main_hash) > -1;
 }
@@ -141,8 +137,8 @@ function do_hashchange_normal(from_reload) {
 }
 
 function do_hashchange_overlay(old_hash) {
-    var base = get_main_hash(window.location.hash);
+    var base = hash_util.get_hash_category(window.location.hash);
-    var old_base = get_main_hash(old_hash);
+    var old_base = hash_util.get_hash_category(old_hash);
 
     var coming_from_overlay = is_overlay_hash(old_hash || '#');