Feat: Implement Function.prototype.call and test case

closes #797

boa/src/builtins/function/mod.rs

@@ -312,6 +312,25 @@ impl BuiltInFunctionObject {
     fn prototype(_: &Value, _: &[Value], _: &mut Context) -> Result<Value> {
         Ok(Value::undefined())
     }
+
+    /// `Function.prototype.call`
+    ///
+    /// The call() method invokes self with the first argument as the `this` value.
+    ///
+    /// More information:
+    ///  - [MDN documentation][mdn]
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-function.prototype.call
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call
+    fn call(this: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
+        let func = this;
+        if !func.is_function() {
+            return context.throw_type_error("TODO: this is not a function");
+        }
+        let this_arg: Value = args.get(0).map_or_else(Value::default, Value::clone);
+        context.call(func, &this_arg, &args[1..])
+    }
 }
 
 impl BuiltIn for BuiltInFunctionObject {
@@ -339,6 +358,7 @@ impl BuiltIn for BuiltInFunctionObject {
         )
         .name(Self::NAME)
         .length(Self::LENGTH)
+        .method(Self::call, "call", 1)
         .build();
 
         (Self::NAME, function_object.into(), Self::attribute())

boa/src/builtins/function/tests.rs

@@ -114,3 +114,15 @@ fn function_prototype_length() {
     assert!(value.is_number());
     assert_eq!(value.as_number().unwrap(), 0.0);
 }
+
+#[test]
+fn function_prototype_call() {
+    let mut engine = Context::new();
+    let func = r#"
+        let e = new Error()
+        Object.prototype.toString.call(e)
+        "#;
+    let value = forward_val(&mut engine, func).unwrap();
+    assert!(value.is_string());
+    assert_eq!(value.as_string().unwrap(), "[object Error]");
+}