Parse JSON string inside mongodb $where function

gistfile1.js

db.collection.find({ 
  $where: function() { 
    var obj = eval("(function(){ return " + this.object + " })()"); 
    if(obj.param) { 
      if(obj.param === "something") { 
        return this 
      } else { 
        return false 
      } 
    } else { 
      return false 
    } 
  } 
}).forEach(function (e, index, array) { printjson(e) });