Change default return type from null to undefined (#643)

* Change default return type from null to undefined

* Change default return value of block

* Add link to spec

* Add tests

boa/src/exec/block/mod.rs

@@ -16,7 +16,9 @@ impl Executable for Block {
             )));
         }
 
-        let mut obj = Value::null();
+        // https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation
+        // The return value is uninitialized, which means it defaults to Value::Undefined
+        let mut obj = Value::default();
         for statement in self.statements() {
             obj = statement.run(interpreter)?;
 

boa/src/exec/statement_list.rs

@@ -6,7 +6,10 @@ use crate::{builtins::value::Value, syntax::ast::node::StatementList, BoaProfile
 impl Executable for StatementList {
     fn run(&self, interpreter: &mut Interpreter) -> Result<Value> {
         let _timer = BoaProfiler::global().start_event("StatementList", "exec");
-        let mut obj = Value::null();
+
+        // https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation
+        // The return value is uninitialized, which means it defaults to Value::Undefined
+        let mut obj = Value::default();
         interpreter.set_current_state(InterpreterState::Executing);
         for (i, item) in self.statements().iter().enumerate() {
             let val = item.run(interpreter)?;

boa/src/exec/tests.rs

@@ -15,6 +15,12 @@ fn function_declaration_returns_undefined() {
     assert_eq!(&exec(scenario), "undefined");
 }
 
+#[test]
+fn empty_function_returns_undefined() {
+    let scenario = "(function () {}) ()";
+    assert_eq!(&exec(scenario), "undefined");
+}
+
 #[test]
 fn property_accessor_member_expression_dot_notation_on_string_literal() {
     let scenario = r#"
@@ -1205,3 +1211,9 @@ fn comma_operator() {
     "#;
     assert_eq!(&exec(scenario), "2");
 }
+
+#[test]
+fn test_result_of_empty_block() {
+    let scenario = "{}";
+    assert_eq!(&exec(scenario), "undefined");
+}