Using a closure or wrapped function

usingAClosure.js

//That's your route
function(req,res,next){
  var username = req.session.username;
  var filteredArray = _.filter(myArray,function(value){
    return value == username; //username is in your closure there...
  });
}

usingAWrappedFunction.js

//That's your route
function(req,res,next){
  var myWrappedIterator = function(username){
   return function(value){ return username == value;}; //That's another closure actually... but in a wrapped function.
  }
  var filteredArray = _.filter(myArray,myWrappedIterator(req.session.username));
}