LibJS: Implement void operator

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/d30db070480
Pull-request: https://github.com/SerenityOS/serenity/pull/1811

Libraries/LibJS/AST.cpp

@@ -391,7 +391,11 @@ Value UnaryExpression::execute(Interpreter& interpreter) const
             return js_string(interpreter, "object");
         case Value::Type::Boolean:
             return js_string(interpreter, "boolean");
+        default:
+            ASSERT_NOT_REACHED();
         }
+    case UnaryOp::Void:
+        return js_undefined();
     }
 
     ASSERT_NOT_REACHED();
@@ -539,6 +543,9 @@ void UnaryExpression::dump(int indent) const
     case UnaryOp::Typeof:
         op_string = "typeof ";
         break;
+    case UnaryOp::Void:
+        op_string = "void ";
+        break;
     }
 
     print_indent(indent);

Libraries/LibJS/AST.h

@@ -395,6 +395,7 @@ enum class UnaryOp {
     Plus,
     Minus,
     Typeof,
+    Void,
 };
 
 class UnaryExpression : public Expression {

Libraries/LibJS/Parser.cpp

@@ -185,6 +185,7 @@ Associativity Parser::operator_associativity(TokenType type) const
     case TokenType::EqualsEqualsEquals:
     case TokenType::ExclamationMarkEqualsEquals:
     case TokenType::Typeof:
+    case TokenType::Void:
     case TokenType::Ampersand:
     case TokenType::Caret:
     case TokenType::Pipe:
@@ -416,6 +417,9 @@ NonnullRefPtr<Expression> Parser::parse_unary_prefixed_expression()
     case TokenType::Typeof:
         consume();
         return create_ast_node<UnaryExpression>(UnaryOp::Typeof, parse_expression(precedence, associativity));
+    case TokenType::Void:
+        consume();
+        return create_ast_node<UnaryExpression>(UnaryOp::Void, parse_expression(precedence, associativity));
     default:
         m_parser_state.m_has_errors = true;
         expected("primary expression (missing switch case)");
@@ -968,7 +972,8 @@ bool Parser::match_unary_prefixed_expression() const
         || type == TokenType::Tilde
         || type == TokenType::Plus
         || type == TokenType::Minus
-        || type == TokenType::Typeof;
+        || type == TokenType::Typeof
+        || type == TokenType::Void;
 }
 
 bool Parser::match_secondary_expression() const

Libraries/LibJS/Tests/void-basic.js

@@ -0,0 +1,20 @@
+load("test-common.js");
+
+try {
+    assert(void "" === undefined);
+    assert(void "foo" === undefined);
+    assert(void 1 === undefined);
+    assert(void 42 === undefined);
+    assert(void true === undefined);
+    assert(void false === undefined);
+    assert(void null === undefined);
+    assert(void undefined === undefined);
+    assert(void function () {} === undefined);
+    assert(void (() => {}) === undefined);
+    assert(void (() => "hello friends")() === undefined);
+    assert((() => void "hello friends")() === undefined);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}