JSON.stringify(undefined) should return undefined (#512)

boa/src/builtins/json/mod.rs

@@ -108,7 +108,9 @@ impl Json {
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
     pub(crate) fn stringify(_: &mut Value, args: &[Value], ctx: &mut Interpreter) -> ResultValue {
         let object = match args.get(0) {
-            Some(obj) if obj.is_symbol() || obj.is_function() => return Ok(Value::undefined()),
+            Some(obj) if obj.is_symbol() || obj.is_function() || obj.is_undefined() => {
+                return Ok(Value::undefined())
+            }
             None => return Ok(Value::undefined()),
             Some(obj) => obj,
         };

boa/src/builtins/json/tests.rs

@@ -184,20 +184,48 @@ fn json_stringify_function_replacer_propogate_error() {
 }
 
 #[test]
-fn json_stringify_return_undefined() {
+fn json_stringify_function() {
     let realm = Realm::create();
     let mut engine = Interpreter::new(realm);
 
-    let actual_no_args = forward(&mut engine, r#"JSON.stringify()"#);
     let actual_function = forward(&mut engine, r#"JSON.stringify(() => {})"#);
-    let actual_symbol = forward(&mut engine, r#"JSON.stringify(Symbol())"#);
     let expected = forward(&mut engine, r#"undefined"#);
 
-    assert_eq!(actual_no_args, expected);
     assert_eq!(actual_function, expected);
+}
+
+#[test]
+fn json_stringify_undefined() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    let actual_undefined = forward(&mut engine, r#"JSON.stringify(undefined)"#);
+    let expected = forward(&mut engine, r#"undefined"#);
+
+    assert_eq!(actual_undefined, expected);
+}
+
+#[test]
+fn json_stringify_symbol() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    let actual_symbol = forward(&mut engine, r#"JSON.stringify(Symbol())"#);
+    let expected = forward(&mut engine, r#"undefined"#);
+
     assert_eq!(actual_symbol, expected);
 }
 
+#[test]
+fn json_stringify_no_args() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    let actual_no_args = forward(&mut engine, r#"JSON.stringify()"#);
+    let expected = forward(&mut engine, r#"undefined"#);
+
+    assert_eq!(actual_no_args, expected);
+}
+
 #[test]
 fn json_parse_array_with_reviver() {
     let realm = Realm::create();