dandycode.js

// Implementation of observable value with cb function
function dandycode() {
  var value = Array.prototype.slice.call(arguments)[0];
  var dandy = {};

  if (!(this instanceof dandycode)) {
    return new dandycode(value);
  }

  this.value = value;
  this.observables = [];

  dandy = {
    set: function (value) { 
      this.observables.forEach(fn => {
        fn.call(this, this.value, value);
      })
      return this.value = value 
    },
    get: function () { 
      return this.value 
    }
  };

  dandy = Object.assign(dandy, this);

  return function () {
    if (arguments[0] === 'observe' 
        && typeof arguments[1] === 'function') {
            this.observables.push(arguments[1]);
    } else {
        return arguments.length === 0 ? dandy.get() : dandy.set(arguments[0]);
    }
  
  }.bind(this);
}

var test = dandycode(20);
var myAge = dandycode(35);

myAge('observe', function(oldVal, newVal) { 
  console.log('Im older now', oldVal, newVal);
});

test('observe', function(oldVal, newVal) { 
  console.log('observer is fired', oldVal, newVal);
});

test('observe', function(oldVal, newVal) { 
  console.log('another observer which multiple new value', newVal * 2);
});

console.log(test());
console.log(test(33));
console.log(test());
console.log(test(25));
console.log(test(2));
console.log(test());
console.log(test(35));
console.log(test());
console.log(myAge());
console.log(myAge(36));