LibJS: Initialize functions in spec order

This is only visible with something like `Object.getOwnPropertyNames` on
the global object. All other declaration instantiations put the
functions on an environment making the order invisible.
Note that spec order is not quite tree order as in non-strict mode
functions which get hoisted out of blocks appear before top level
functions.

Co-authored-by: Hendiadyoin1 <leon.a@serenityos.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp b/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
index 1f108dca5d..39abfd7172 100644
--- a/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
+++ b/Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
@@ -804,6 +804,8 @@ ThrowCompletionOr<void> eval_declaration_instantiation(VM& vm, Program const& pr
         // Note: Already done in step iv.
 
         // 3. Insert d as the first element of functionsToInitialize.
+        // NOTE: Since prepending is much slower, we just append
+        //       and iterate in reverse order in step 17 below.
         functions_to_initialize.append(function);
         return {};
     }));
@@ -958,7 +960,10 @@ ThrowCompletionOr<void> eval_declaration_instantiation(VM& vm, Program const& pr
     }));
 
     // 17. For each Parse Node f of functionsToInitialize, do
-    for (auto& declaration : functions_to_initialize) {
+    // NOTE: We iterate in reverse order since we appended the functions
+    //       instead of prepending. We append because prepending is much slower
+    //       and we only use the created vector here.
+    for (auto& declaration : functions_to_initialize.in_reverse()) {
         // a. Let fn be the sole element of the BoundNames of f.
         // b. Let fo be InstantiateFunctionObject of f with arguments lexEnv and privateEnv.
         auto* function = ECMAScriptFunctionObject::create(realm, declaration.name(), declaration.source_text(), declaration.body(), declaration.parameters(), declaration.function_length(), lexical_environment, private_environment, declaration.kind(), declaration.is_strict_mode(), declaration.might_need_arguments_object());