<h1>Need Updates about Screen Width?</h1>
<p>I am wider than 599px now.</p>

body {
  text-align: center;
  margin: 50px;
  font-family: 'Lato';
}

h1 {
  font-size: 2rem;
  text-transform: uppercase;
  color: green;
  font-weight: 900;
}

p {
  font-size: 1.5rem;
}

@media (min-width: 600px) {
  h1 {
    font-size: 4rem;
  }
}

let myMediaQuery = window.matchMedia('(min-width: 600px)');

function widthChangeCallback(myMediaQuery) {
  if(myMediaQuery.matches) {
    document.querySelector("p").textContent = "I am wider than 599px now.";
   } else {
     document.querySelector("p").textContent = "I am narrower than 599px now.";
   }
}

myMediaQuery.addEventListener('change', widthChangeCallback);

widthChangeCallback(myMediaQuery);