Простой пример observable

index.html

<input type="text" id="a-text">
+
<input type="text" id="b-text">
=
<input type="text" id="c-text" readonly>

observable.js

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(); });

  input.addEventListener('input', function() {
    observable(input.value);
  });
}

var aText = document.getElementById('a-text');
var bText = document.getElementById('b-text');
var cText = document.getElementById('c-text');

var a = observable("Hello, "), b = observable("World!");
var c = computed(function(){ return a() + b(); }, [a, b]);

bindValue(aText, a);
bindValue(bText, b);
bindValue(cText, c);