lrud-other.js

var Rx = require('rx'),
    log = console.log.bind(console);
    
Rx.window = Rx; // #hack to make rx-dom work
require('rx-dom'); 

var prop = 'which',
    map = {
      37: 'west',
      38: 'north',
      39: 'east',
      40: 'south'
    };

Rx.DOM.fromEvent(document.body, 'keydown')
  .map(function (e) { return e.which; })
  .filter(function (which) {
    return map.hasOwnProperty(which);
  })
  .map(function (key) { return map[key]; })
  .subscribe(log);

lrud.js

var Rx = require('rx'),
    log = console.log.bind(console);
    
Rx.window = Rx; // #hack to make rx-dom work
require('rx-dom'); 

Rx.DOM.fromEvent(document.body, 'keydown')
  .pluckMap('which', {
    37: 'west',
    38: 'north',
    39: 'east',
    40: 'south'
  })
  .subscribe(log);

pluckMap.js

Rx.Observable.prototype.pluckMap = function (prop, obj) {
  var self = this;
  return Rx.Observable.create(function (o) {
    return self.subscribe(onNext, o.onError, o.onCompleted);
    function onNext (x) {
      var key = x[prop];
      if (obj.hasOwnProperty(key)) {
        o.onNext(obj[key]);
      }
    }
  });
};