LibJS: Add a fast path for creating per-iteration DeclarativeEnvironment

The steps for creating a DeclarativeEnvironment for each iteration of a
for-loop can be done equivalently to the spec without following the spec
directly. For each binding creating in the loop's init expression, we:

    1. Create a new binding in the new environment.
    2. Grab the current value of the binding in the old environment.
    3. Set the value in the new environment to the old value.

This can be replaced by initializing the bindings vector in the new
environment directly with the bindings in the old environment (but only
copying the bindings of the init statement).

1 files changed, 14 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp b/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
index ec61b3ca13..e3255d6e0d 100644
--- a/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
+++ b/Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp
@@ -13,6 +13,14 @@
 
 namespace JS {
 
+DeclarativeEnvironment* DeclarativeEnvironment::create_for_per_iteration_bindings(Badge<ForStatement>, DeclarativeEnvironment& other, size_t bindings_size)
+{
+    auto bindings = other.m_bindings.span().slice(0, bindings_size);
+    auto* parent_scope = other.outer_environment();
+
+    return parent_scope->heap().allocate_without_global_object<DeclarativeEnvironment>(parent_scope, bindings);
+}
+
 DeclarativeEnvironment::DeclarativeEnvironment()
     : Environment(nullptr)
 {
@@ -23,6 +31,12 @@ DeclarativeEnvironment::DeclarativeEnvironment(Environment* parent_scope)
 {
 }
 
+DeclarativeEnvironment::DeclarativeEnvironment(Environment* parent_scope, Span<Binding const> bindings)
+    : Environment(parent_scope)
+    , m_bindings(bindings)
+{
+}
+
 DeclarativeEnvironment::~DeclarativeEnvironment()
 {
 }