Almost working

app.js

var fs = require('fs');

var data = function(dataFile, mapFile, done) {
  fs.readFile(dataFile, function(err, dataString) {
    if(err) return done(err);

    var evaluated = eval(dataString+";data");
    
    var data = evaluated.reduce(function(data, item) {
      data[item.key] = item.val;
      return data;
    }, {});

    fs.readFile(mapFile, function(err, mapString) {
      if(err) return done(err);

      var map = JSON.parse(mapString, function(key, value) {
        if(value in data) return data[value];
        return value;
      });

      return done(null, map);
    });
  });
};

data('data.json', 'map.json', function(err, map) {
  if (err) throw err;

  console.log(JSON.stringify(map));
});