Edit in JSFiddle

function createObjectWithPrivacy(){
	let private = 42;
  
  return {
  	public: 'public',
    guessMySecret(guess) {
      return guess === private;
    }
  };
}

let o = createObjectWithPrivacy();
document.getElementById('out1').textContent = o.guessMySecret('a guess');

<dic id='out1'></dic>