observable.js

function observable(value) {
  let listeners = [];
  
  let notify = (newValue) => listeners.forEach((listener) => listener(newValue));
  
  function accessor(newValue) {
    if(arguments.length && newValue !== value) {
      value = newValue;
      notify(newValue);
    }
    
    return value;
  }
  
  accessor.subscribe = (listener) => listeners.push(listener);
  

 return accessor;
}

var n = observable(4);

n.subscribe( val => console.log('subscribe val: ' + val) );
n.subscribe( val => console.log('subscribe again val: ' + val) );

n(20);
n(25);