LibJS: Implement Object.prototype.isPrototypeOf

Spec: https://tc39.es/ecma262/#sec-object.prototype.isprototypeof

4 files changed, 40 insertions, 0 deletions

diff --git a/Libraries/LibJS/Runtime/CommonPropertyNames.h b/Libraries/LibJS/Runtime/CommonPropertyNames.h
index 3e92dce93c..f6d96fa552 100644
--- a/Libraries/LibJS/Runtime/CommonPropertyNames.h
+++ b/Libraries/LibJS/Runtime/CommonPropertyNames.h
@@ -149,6 +149,7 @@ namespace JS {
     P(isFinite)                              \
     P(isInteger)                             \
     P(isNaN)                                 \
+    P(isPrototypeOf)                         \
     P(isSafeInteger)                         \
     P(isView)                                \
     P(join)                                  \
diff --git a/Libraries/LibJS/Runtime/ObjectPrototype.cpp b/Libraries/LibJS/Runtime/ObjectPrototype.cpp
index f5cd0604b1..b580769980 100644
--- a/Libraries/LibJS/Runtime/ObjectPrototype.cpp
+++ b/Libraries/LibJS/Runtime/ObjectPrototype.cpp
@@ -50,6 +50,7 @@ void ObjectPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.toLocaleString, to_locale_string, 0, attr);
     define_native_function(vm.names.valueOf, value_of, 0, attr);
     define_native_function(vm.names.propertyIsEnumerable, property_is_enumerable, 1, attr);
+    define_native_function(vm.names.isPrototypeOf, is_prototype_of, 1, attr);
 }
 
 ObjectPrototype::~ObjectPrototype()
@@ -138,4 +139,23 @@ JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::property_is_enumerable)
     return Value(property_descriptor.value().attributes.is_enumerable());
 }
 
+JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::is_prototype_of)
+{
+    auto object_argument = vm.argument(0);
+    if (!object_argument.is_object())
+        return Value(false);
+    auto* object = &object_argument.as_object();
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return {};
+
+    for (;;) {
+        object = object->prototype();
+        if (!object)
+            return Value(false);
+        if (same_value(this_object, object))
+            return Value(true);
+    }
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/ObjectPrototype.h b/Libraries/LibJS/Runtime/ObjectPrototype.h
index 325c6c9c79..a68857e8ae 100644
--- a/Libraries/LibJS/Runtime/ObjectPrototype.h
+++ b/Libraries/LibJS/Runtime/ObjectPrototype.h
@@ -46,6 +46,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
     JS_DECLARE_NATIVE_FUNCTION(property_is_enumerable);
+    JS_DECLARE_NATIVE_FUNCTION(is_prototype_of);
 };
 
 }
diff --git a/Libraries/LibJS/Tests/builtins/Object/Object.prototype.isPrototypeOf.js b/Libraries/LibJS/Tests/builtins/Object/Object.prototype.isPrototypeOf.js
new file mode 100644
index 0000000000..f3f82174d2
--- /dev/null
+++ b/Libraries/LibJS/Tests/builtins/Object/Object.prototype.isPrototypeOf.js
@@ -0,0 +1,18 @@
+test("basic functionality", () => {
+    function A() {}
+    function B() {}
+
+    A.prototype = new B();
+    const C = new A();
+
+    expect(A.prototype.isPrototypeOf(C)).toBeTrue();
+    expect(B.prototype.isPrototypeOf(C)).toBeTrue();
+
+    expect(A.isPrototypeOf(C)).toBeFalse();
+    expect(B.isPrototypeOf(C)).toBeFalse();
+
+    const D = new Object();
+    expect(Object.prototype.isPrototypeOf(D)).toBeTrue();
+    expect(Function.prototype.isPrototypeOf(D.toString)).toBeTrue();
+    expect(Array.prototype.isPrototypeOf([])).toBeTrue();
+});