code: function(allowIn) {
        var node = this.startNode(_SequenceExpression);
        var expr = this.parseAssignmentExpression(allowIn);
        if (this.value !== ',') {
          return expr;
        }
  
        var exprs = [expr];
  
        do {
          this.next();
          exprs[exprs.length] = this.parseAssignmentExpression(allowIn);
        } while (this.value === ',');
  
        node.expressions = exprs;
        return this.finishNode(node);
      }
docs: ECMA-262 A.2 Expressions