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