LibJS: Add the Function.[[ThisMode]] field

This is not a behavioral change in itself, just prep work for future
spec-compliance changes.

2 files changed, 18 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Function.h b/Userland/Libraries/LibJS/Runtime/Function.h
index f8298515bb..997e5e2bde 100644
--- a/Userland/Libraries/LibJS/Runtime/Function.h
+++ b/Userland/Libraries/LibJS/Runtime/Function.h
@@ -47,6 +47,16 @@ public:
     // Used as the outer environment when evaluating the code of the function.
     virtual EnvironmentRecord* environment() { return nullptr; }
 
+    enum class ThisMode : u8 {
+        Lexical,
+        Strict,
+        Global,
+    };
+
+    // [[ThisMode]]
+    ThisMode this_mode() const { return m_this_mode; }
+    void set_this_mode(ThisMode this_mode) { m_this_mode = this_mode; }
+
 protected:
     virtual void visit_edges(Visitor&) override;
 
@@ -59,6 +69,7 @@ private:
     Vector<Value> m_bound_arguments;
     Value m_home_object;
     ConstructorKind m_constructor_kind = ConstructorKind::Base;
+    ThisMode m_this_mode { ThisMode::Global };
 };
 
 }
diff --git a/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp b/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
index 523b6cdbd3..633609d87b 100644
--- a/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
@@ -61,6 +61,13 @@ ScriptFunction::ScriptFunction(GlobalObject& global_object, const FlyString& nam
     , m_is_strict(is_strict)
     , m_is_arrow_function(is_arrow_function)
 {
+    // NOTE: This logic is from OrdinaryFunctionCreate, https://tc39.es/ecma262/#sec-ordinaryfunctioncreate
+    if (m_is_arrow_function)
+        set_this_mode(ThisMode::Lexical);
+    else if (m_is_strict)
+        set_this_mode(ThisMode::Strict);
+    else
+        set_this_mode(ThisMode::Global);
 }
 
 void ScriptFunction::initialize(GlobalObject& global_object)