From 93cd17db74a62026aa3f4aaca4c0e5c1bc96fb15 Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Tue, 3 Jun 2025 19:09:33 +0200
Subject: [PATCH] LibJS: Add fast path `internal_has_property()` for Array

If array has packed index property storage without holes, we could check
if indexed property is present simple by checking if it's less than
array's length.

Makes the following program go 1.1x faster:
```js
function f() {
    let array = [];
    for (let i = 0; i < 3_000; i++) {
        array.push(i);
    }

    for (let i = 0; i < 10_000; i++) {
        array.map(x => x * 2);
    }
}

f();
```
---
 Libraries/LibJS/Runtime/Array.cpp | 13 +++++++++++++
 Libraries/LibJS/Runtime/Array.h   |  1 +
 2 files changed, 14 insertions(+)

diff --git a/Libraries/LibJS/Runtime/Array.cpp b/Libraries/LibJS/Runtime/Array.cpp
index 4cb10429406..f5ead14928f 100644
--- a/Libraries/LibJS/Runtime/Array.cpp
+++ b/Libraries/LibJS/Runtime/Array.cpp
@@ -393,6 +393,19 @@ ThrowCompletionOr<bool> Array::internal_define_own_property(PropertyKey const& p
     return Object::internal_define_own_property(property_key, property_descriptor, precomputed_get_own_property);
 }
 
+// NON-STANDARD: Fast path to quickly check if an indexed property exists in array without holes
+ThrowCompletionOr<bool> Array::internal_has_property(PropertyKey const& property_key) const
+{
+    auto const* storage = indexed_properties().storage();
+    if (property_key.is_number() && !m_is_proxy_target && storage && storage->is_simple_storage()) {
+        auto const& simple_storage = static_cast<SimpleIndexedPropertyStorage const&>(*storage);
+        if (!simple_storage.has_empty_elements() && property_key.as_number() < simple_storage.array_like_size()) {
+            return true;
+        }
+    }
+    return Object::internal_has_property(property_key);
+}
+
 // NON-STANDARD: Used to reject deletes to ephemeral (non-configurable) length property
 ThrowCompletionOr<bool> Array::internal_delete(PropertyKey const& property_key)
 {
diff --git a/Libraries/LibJS/Runtime/Array.h b/Libraries/LibJS/Runtime/Array.h
index cf936272709..c0997ba090c 100644
--- a/Libraries/LibJS/Runtime/Array.h
+++ b/Libraries/LibJS/Runtime/Array.h
@@ -50,6 +50,7 @@ public:
     virtual ThrowCompletionOr<Optional<PropertyDescriptor>> internal_get_own_property(PropertyKey const&) const override final;
     virtual ThrowCompletionOr<bool> internal_set(PropertyKey const&, Value value, Value receiver, CacheablePropertyMetadata*, PropertyLookupPhase) override;
     virtual ThrowCompletionOr<bool> internal_define_own_property(PropertyKey const&, PropertyDescriptor const&, Optional<PropertyDescriptor>* precomputed_get_own_property = nullptr) override final;
+    virtual ThrowCompletionOr<bool> internal_has_property(PropertyKey const&) const override final;
     virtual ThrowCompletionOr<bool> internal_delete(PropertyKey const&) override;
     virtual ThrowCompletionOr<GC::RootVector<Value>> internal_own_property_keys() const override final;