code: function parseArrayInitialiser() {
          var elements = [],
              undef;
  
          expect('[');
  
          while (!match(']')) {
              if (match(',')) {
                  lex();
                  elements.push(undef);
              } else {
                  elements.push(parseAssignmentExpression());
  
                  if (!match(']')) {
                      expect(',');
                  }
              }
          }
  
          expect(']');
  
          return {
              type: Syntax.ArrayExpression,
              elements: elements
          };
      }
docs: 11.1.4 Array Initialiser