Make `Function.prototype` a function (#802)

boa/src/builtins/function/mod.rs

@@ -14,7 +14,7 @@
 use crate::{
     builtins::{Array, BuiltIn},
     environment::lexical_environment::Environment,
-    object::{ConstructorBuilder, Object, ObjectData, PROTOTYPE},
+    object::{ConstructorBuilder, FunctionBuilder, Object, ObjectData, PROTOTYPE},
     property::{Attribute, Property},
     syntax::ast::node::{FormalParameter, RcStatementList},
     BoaProfiler, Context, Result, Value,
@@ -301,13 +301,17 @@ pub struct BuiltInFunctionObject;
 impl BuiltInFunctionObject {
     pub const LENGTH: usize = 1;
 
-    fn constructor(this: &Value, _args: &[Value], _context: &mut Context) -> Result<Value> {
+    fn constructor(this: &Value, _: &[Value], _: &mut Context) -> Result<Value> {
         this.set_data(ObjectData::Function(Function::BuiltIn(
             BuiltInFunction(|_, _, _| Ok(Value::undefined())),
             FunctionFlags::CALLABLE | FunctionFlags::CONSTRUCTABLE,
         )));
         Ok(this.clone())
     }
+
+    fn prototype(_: &Value, _: &[Value], _: &mut Context) -> Result<Value> {
+        Ok(Value::undefined())
+    }
 }
 
 impl BuiltIn for BuiltInFunctionObject {
@@ -320,6 +324,14 @@ impl BuiltIn for BuiltInFunctionObject {
     fn init(context: &mut Context) -> (&'static str, Value, Attribute) {
         let _timer = BoaProfiler::global().start_event("function", "init");
 
+        let function_prototype = context.standard_objects().function_object().prototype();
+        FunctionBuilder::new(context, Self::prototype)
+            .name("")
+            .length(0)
+            .callable(true)
+            .constructable(false)
+            .build_function_prototype(&function_prototype);
+
         let function_object = ConstructorBuilder::with_standard_object(
             context,
             Self::constructor,

boa/src/builtins/function/tests.rs

@@ -61,3 +61,56 @@ fn self_mutating_function_when_constructing() {
         3
     );
 }
+
+#[test]
+fn call_function_prototype() {
+    let mut engine = Context::new();
+    let func = r#"
+        Function.prototype()
+        "#;
+    let value = forward_val(&mut engine, func).unwrap();
+    assert!(value.is_undefined());
+}
+
+#[test]
+fn call_function_prototype_with_arguments() {
+    let mut engine = Context::new();
+    let func = r#"
+        Function.prototype(1, "", new String(""))
+        "#;
+    let value = forward_val(&mut engine, func).unwrap();
+    assert!(value.is_undefined());
+}
+
+#[test]
+fn call_function_prototype_with_new() {
+    let mut engine = Context::new();
+    let func = r#"
+        new Function.prototype()
+        "#;
+    let value = forward_val(&mut engine, func);
+    assert!(value.is_err());
+}
+
+#[test]
+fn function_prototype_name() {
+    let mut engine = Context::new();
+    let func = r#"
+        Function.prototype.name
+        "#;
+    let value = forward_val(&mut engine, func).unwrap();
+    assert!(value.is_string());
+    assert!(value.as_string().unwrap().is_empty());
+}
+
+#[test]
+#[allow(clippy::float_cmp)]
+fn function_prototype_length() {
+    let mut engine = Context::new();
+    let func = r#"
+        Function.prototype.length
+        "#;
+    let value = forward_val(&mut engine, func).unwrap();
+    assert!(value.is_number());
+    assert_eq!(value.as_number().unwrap(), 0.0);
+}

boa/src/object/mod.rs

@@ -670,6 +670,29 @@ impl<'context> FunctionBuilder<'context> {
 
         GcObject::new(function)
     }
+
+    /// Initializes the `Function.prototype` function object.
+    pub(crate) fn build_function_prototype(&mut self, object: &GcObject) {
+        let mut object = object.borrow_mut();
+        object.data = ObjectData::Function(Function::BuiltIn(
+            self.function,
+            FunctionFlags::from_parameters(self.callable, self.constructable),
+        ));
+        object.set_prototype_instance(
+            self.context
+                .standard_objects()
+                .object_object()
+                .prototype()
+                .into(),
+        );
+        let attribute = Attribute::READONLY | Attribute::NON_ENUMERABLE | Attribute::PERMANENT;
+        if let Some(name) = self.name.take() {
+            object.insert_property("name", name, attribute);
+        } else {
+            object.insert_property("name", "", attribute);
+        }
+        object.insert_property("length", self.length, attribute);
+    }
 }
 
 /// Builder for creating objects with properties.