function observable(value) {

  var listeners = [];

​

  function notify(newValue) {

    listeners.forEach(function(listener){ listener(newValue); });

  }

​

  function accessor(newValue) {

    if (arguments.length && newValue !== value) {

      value = newValue;

      notify(newValue);

    }

    return value;

  }

​

  accessor.subscribe = function(listener) { listeners.push(listener); };

​

  return accessor;

}

​

function computed(calculation, dependencies) {

  var value = observable(calculation());

​

  function listener(v) {value(calculation()); }

  dependencies.forEach(function(dependency) {

    dependency.subscribe(listener);

  });

​

  function getter() { return value(); }

  getter.subscribe = value.subscribe;

​

  return getter;

}

​

function bindValue(input, observable) {

  var initial = observable();

  input.value = initial;

  observable.subscribe(function(){ input.value = observable(); });

​

  var converter = function(v) { return v; };

  if (typeof initial == 'number') {

    converter = function(n){ return isNaN(n = parseFloat(n)) ? 0 : n; };

  }

​

  input.addEventListener('input', function() {

    observable(converter(input.value));

  });

}

​

var aText = document.getElementById('a-text');

var bText = document.getElementById('b-text');

var cText = document.getElementById('c-text');

​

var a = observable(3), b = observable(2);

var c = computed(function(){ return a() + b(); }, [a, b]);

​

bindValue(aText, a);

bindValue(bText, b);

bindValue(cText, c);