Passing parameters to PEG.js Parser

grammar.pegjs

{
    var thisParser = this;
    
   function resolveSymbol(parser, sym){
      return (parser.symbols || {})[sym];
   }

   function flatten(array){
      var flat = [];
      for (var i = 0, l = array.length; i < l; i++){
          var type = Object.prototype.toString.call(array[i]).split(' ').pop().split(']').shift().toLowerCase();
          if (type) { flat = flat.concat(/^(array|collection|arguments|object)$/.test(type) ? flatten(array[i]) : array[i]); }
      }
      return flat;
   }

}
start 
  = Variable

Variable
 = symbol:([A-z_\$][A-z0-9_\$]*) {

  return {
           value: resolveSymbol(thisParser, flatten(symbol).join(""))
        }
 }

interpreter.js

var FileSystem   = require('fs');
var Path         = require('path');
var PegJs        = require("pegjs");

var grammar = FileSystem.readFileSync(Path.resolve(__dirname, "./grammar.pegjs")).toString();
var parser  = PegJs.buildParser(grammar).toString());

parser.symbols = {x: 5};
parser.parse("x");