Cannot access private property with Set method in Symfony

function MyClass() {
  var secret = "foo";
  this.tellSecret = function() {
    return secret;
  };
}

var inst = new MyClass();
console.log(inst.tellSecret()); // => "foo"
console.log(inst.secret);       // => undefined

function MyClass() {
  this._secret = "foo";
}

MyClass.prototype.tellSecret = function() {
  return this._secret;
};

function foo(){
    this.a = 123;

    this.b();
}

foo.prototype.b = function b(){
    alert(this.a);
}

var o = new foo();
o.a = 456;

$description = $jobDetails->getJobDescription();

public int Num { private get; set; }