Fully implement EmptyStatement (#1151)

* add EmptyStatement

* add comments

* format code

* add test

* add test

* fmt

boa/src/exec/tests.rs

@@ -1519,3 +1519,13 @@ fn test_strict_mode_dup_func_parameters() {
 
     assert!(string.starts_with("Uncaught \"SyntaxError\": "));
 }
+
+#[test]
+fn test_empty_statement() {
+    let src = r#"
+        ;;;let a = 10;;
+        if(a) ;
+        a
+    "#;
+    assert_eq!(&exec(src), "10");
+}

boa/src/syntax/ast/node/mod.rs

@@ -196,6 +196,18 @@ pub enum Node {
 
     /// A 'while {...}' node. [More information](./iteration/struct.WhileLoop.html).
     WhileLoop(WhileLoop),
+
+    /// A empty node.
+    ///
+    /// Empty statement do nothing, just return undefined.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#prod-EmptyStatement
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Empty
+    Empty,
 }
 
 impl Display for Node {
@@ -274,6 +286,7 @@ impl Node {
             Self::AsyncFunctionDecl(ref decl) => decl.display(f, indentation),
             Self::AsyncFunctionExpr(ref expr) => expr.display(f, indentation),
             Self::AwaitExpr(ref expr) => expr.display(f, indentation),
+            Self::Empty => write!(f, ";"),
         }
     }
 }
@@ -338,6 +351,7 @@ impl Executable for Node {
             Node::Try(ref try_node) => try_node.run(context),
             Node::Break(ref break_node) => break_node.run(context),
             Node::Continue(ref continue_node) => continue_node.run(context),
+            Node::Empty => Ok(Value::Undefined),
         }
     }
 }

boa/src/syntax/parser/statement/mod.rs

@@ -178,6 +178,11 @@ where
                     .parse(cursor)
                     .map(Node::from)
             }
+            TokenKind::Punctuator(Punctuator::Semicolon) => {
+                // parse the EmptyStatement
+                cursor.next().expect("semicolon disappeared");
+                Ok(Node::Empty)
+            }
             TokenKind::Identifier(_) => {
                 // Labelled Statement check
                 cursor.set_goal(InputElement::Div);

boa/src/syntax/parser/tests.rs

@@ -4,8 +4,8 @@ use super::Parser;
 use crate::syntax::ast::{
     node::{
         field::GetConstField, ArrowFunctionDecl, Assign, BinOp, Call, FormalParameter,
-        FunctionDecl, Identifier, LetDecl, LetDeclList, New, Node, Return, StatementList, UnaryOp,
-        VarDecl, VarDeclList,
+        FunctionDecl, Identifier, If, LetDecl, LetDeclList, New, Node, Return, StatementList,
+        UnaryOp, VarDecl, VarDeclList,
     },
     op::{self, CompOp, LogOp, NumOp},
     Const,
@@ -292,3 +292,22 @@ fn spread_in_arrow_function() {
         ],
     );
 }
+
+#[test]
+fn empty_statement() {
+    check_parser(
+        r"
+            ;;var a = 10;
+            if(a) ;
+        ",
+        vec![
+            Node::Empty,
+            VarDeclList::from(vec![VarDecl::new("a", Node::from(Const::from(10)))]).into(),
+            Node::If(If::new::<_, _, Node, _>(
+                Identifier::from("a"),
+                Node::Empty,
+                None,
+            )),
+        ],
+    );
+}