Implement Object.prototype.isPrototypeOf

boa/src/builtins/object/mod.rs

@@ -51,6 +51,7 @@ impl BuiltIn for Object {
         .method(Self::has_own_property, "hasOwnProperty", 0)
         .method(Self::property_is_enumerable, "propertyIsEnumerable", 0)
         .method(Self::to_string, "toString", 0)
+        .method(Self::is_prototype_of, "isPrototypeOf", 0)
         .static_method(Self::create, "create", 2)
         .static_method(Self::set_prototype_of, "setPrototypeOf", 2)
         .static_method(Self::get_prototype_of, "getPrototypeOf", 1)
@@ -258,6 +259,34 @@ impl Object {
         Ok(obj)
     }
 
+    /// `Object.prototype.isPrototypeOf( proto )`
+    ///
+    /// Check whether or not an object exists within another object's prototype chain.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-object.prototype.isprototypeof
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isPrototypeOf
+    pub fn is_prototype_of(this: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
+        let undefined = Value::undefined();
+        let mut v = args.get(0).unwrap_or(&undefined).clone();
+        if !v.is_object() {
+            return Ok(Value::Boolean(false));
+        }
+        let o = Value::from(this.to_object(context)?);
+        loop {
+            v = Self::get_prototype_of(this, &[v], context)?;
+            if v.is_null() {
+                return Ok(Value::Boolean(false));
+            }
+            if same_value(&o, &v) {
+                return Ok(Value::Boolean(true));
+            }
+        }
+    }
+
     /// Define a property in an object
     pub fn define_property(_: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
         let obj = args.get(0).expect("Cannot get object");

boa/src/builtins/object/tests.rs

@@ -279,3 +279,14 @@ fn object_define_properties() {
 
     assert_eq!(forward(&mut context, "obj.p"), "42");
 }
+
+#[test]
+fn object_is_prototype_of() {
+    let mut context = Context::new();
+
+    let init = r#"
+        Object.prototype.isPrototypeOf(String.prototype)
+    "#;
+
+    assert_eq!(context.eval(init).unwrap(), Value::boolean(true));
+}

boa/src/object/gcobject.rs

@@ -198,7 +198,21 @@ impl GcObject {
     // <https://tc39.es/ecma262/#sec-ecmascript-function-objects-construct-argumentslist-newtarget>
     #[track_caller]
     pub fn construct(&self, args: &[Value], context: &mut Context) -> Result<Value> {
-        let this: Value = Object::create(self.get(&PROTOTYPE.into())).into();
+        // If the prototype of the constructor is not an object, then use the default object
+        // prototype as prototype for the new object
+        // see <https://tc39.es/ecma262/#sec-ordinarycreatefromconstructor>
+        // see <https://tc39.es/ecma262/#sec-getprototypefromconstructor>
+        let proto = self.get(&PROTOTYPE.into());
+        let proto = if proto.is_object() {
+            proto
+        } else {
+            context
+                .standard_objects()
+                .object_object()
+                .prototype()
+                .into()
+        };
+        let this: Value = Object::create(proto).into();
 
         let this_function_object = self.clone();
         let body = if let Some(function) = self.borrow().as_function() {