LibJS: Implement the "instanceof" operator

This operator walks the prototype chain of the RHS value and looks for
a "prototype" property with the same value as the prototype of the LHS.

This is pretty cool. :^)

1 files changed, 22 insertions, 0 deletions

diff --git a/Libraries/LibJS/Runtime/Value.cpp b/Libraries/LibJS/Runtime/Value.cpp
index 66582e4622..c36d358213 100644
--- a/Libraries/LibJS/Runtime/Value.cpp
+++ b/Libraries/LibJS/Runtime/Value.cpp
@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/FlyString.h>
 #include <AK/String.h>
 #include <LibJS/Heap/Heap.h>
 #include <LibJS/Runtime/Object.h>
@@ -253,6 +254,27 @@ Value eq(Value lhs, Value rhs)
     return Value(false);
 }
 
+Value instance_of(Value lhs, Value rhs)
+{
+    if (!lhs.is_object() || !rhs.is_object())
+        return Value(false);
+
+    auto* instance_prototype = lhs.as_object()->prototype();
+
+    if (!instance_prototype)
+        return Value(false);
+
+    for (auto* constructor_object = rhs.as_object(); constructor_object; constructor_object = constructor_object->prototype()) {
+        auto prototype_property = constructor_object->get_own_property(*constructor_object, "prototype");
+        if (!prototype_property.has_value())
+            continue;
+        if (prototype_property.value().is_object() && prototype_property.value().as_object() == instance_prototype)
+            return Value(true);
+    }
+
+    return Value(false);
+}
+
 const LogStream& operator<<(const LogStream& stream, const Value& value)
 {
     return stream << value.to_string();