Fix: Make Object.toString() spec compliant*

Test: Add testing of Object.toString(), need update once
Function.prototype.call is implemented

* not fully because Arguments Exotic Objects are not yet implemented

boa/src/builtins/object/mod.rs

@@ -115,9 +115,33 @@ impl Object {
     /// [spec]: https://tc39.es/ecma262/#sec-object.prototype.tostring
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString
     #[allow(clippy::wrong_self_convention)]
-    pub fn to_string(_: &Value, _: &[Value], _: &mut Context) -> Result<Value> {
+    pub fn to_string(this: &Value, _: &[Value], ctx: &mut Context) -> Result<Value> {
-        // FIXME: it should not display the object.
+        if this.is_undefined() {
-        Ok("[object Object]".into())
+            Ok("[object Undefined]".into())
+        } else if this.is_null() {
+            Ok("[object Null]".into())
+        } else {
+            let gc_o = this.to_object(ctx)?;
+            let o = gc_o.borrow();
+            let builtin_tag = match &o.data {
+                ObjectData::Array => "Array",
+                // TODO: Arguments Exotic Objects are currently not supported
+                ObjectData::Function(_) => "Function",
+                ObjectData::Error => "Error",
+                ObjectData::Boolean(_) => "Boolean",
+                ObjectData::Number(_) => "Number",
+                ObjectData::String(_) => "String",
+                ObjectData::Date(_) => "Date",
+                ObjectData::RegExp(_) => "RegExp",
+                _ => "Object",
+            };
+
+            let tag = o.get(&ctx.well_known_symbols().to_string_tag_symbol().into());
+
+            let tag_str = tag.as_string().map(|s| s.as_str()).unwrap_or(builtin_tag);
+
+            Ok(format!("[object {}]", tag_str).into())
+        }
     }
 
     /// `Object.prototype.hasOwnPrototype( property )`

boa/src/builtins/object/tests.rs

@@ -55,7 +55,7 @@ fn object_create_with_number() {
 
 #[test]
 #[ignore]
-// to test on __proto__ somehow. __proto__ getter is not working as expected currently
+// TODO: to test on __proto__ somehow. __proto__ getter is not working as expected currently
 fn object_create_with_function() {
     let mut engine = Context::new();
 
@@ -135,3 +135,43 @@ fn object_property_is_enumerable() {
     );
     assert_eq!(forward(&mut engine, r#"x.propertyIsEnumerable()"#), "false",)
 }
+
+#[test]
+fn object_to_string() {
+    let mut ctx = Context::new();
+    let init = r#"
+        let u = undefined;
+        let n = null;
+        let a = [];
+        Array.prototype.toString = Object.prototype.toString;
+        let f = () => {};
+        Function.prototype.toString = Object.prototype.toString;
+        let b = Boolean();
+        Boolean.prototype.toString = Object.prototype.toString;
+        let i = Number(42);
+        Number.prototype.toString = Object.prototype.toString;
+        let s = String('boa');
+        String.prototype.toString = Object.prototype.toString;
+        let d = new Date(Date.now());
+        Date.prototype.toString = Object.prototype.toString;
+        let re = /boa/;
+        RegExp.prototype.toString = Object.prototype.toString;
+    "#;
+    eprintln!("{}", forward(&mut ctx, init));
+    // TODO: need Function.prototype.call to be implemented
+    // assert_eq!(
+    //     forward(&mut ctx, "Object.prototype.toString.call(u)"),
+    //     "\"[object Undefined]\""
+    // );
+    // assert_eq!(
+    //     forward(&mut ctx, "Object.prototype.toString.call(n)"),
+    //     "\"[object Null]\""
+    // );
+    assert_eq!(forward(&mut ctx, "a.toString()"), "\"[object Array]\"");
+    assert_eq!(forward(&mut ctx, "f.toString()"), "\"[object Function]\"");
+    assert_eq!(forward(&mut ctx, "b.toString()"), "\"[object Boolean]\"");
+    assert_eq!(forward(&mut ctx, "i.toString()"), "\"[object Number]\"");
+    assert_eq!(forward(&mut ctx, "s.toString()"), "\"[object String]\"");
+    assert_eq!(forward(&mut ctx, "d.toString()"), "\"[object Date]\"");
+    assert_eq!(forward(&mut ctx, "re.toString()"), "\"[object RegExp]\"");
+}