code: function parseExpression() {
          var expr = parseAssignmentExpression();
  
          if (match(',')) {
              expr = {
                  type: Syntax.SequenceExpression,
                  expressions: [ expr ]
              };
  
              while (index < length) {
                  if (!match(',')) {
                      break;
                  }
                  lex();
                  expr.expressions.push(parseAssignmentExpression());
              }
  
          }
          return expr;
      }
docs: 11.14 Comma Operator