MediaQueryListEvent: свойство matches

const para = document.querySelector("p"); // This is the UI element where to display the text
const mql = window.matchMedia("(max-width: 600px)");

mql.addEventListener("change", (event) => {
  if (event.matches) {
    // The viewport is 600 pixels wide or less
    para.textContent = "This is a narrow screen — less than 600px wide.";
    document.body.style.backgroundColor = "red";
  } else {
    // The viewport is more than 600 pixels wide
    para.textContent = "This is a wide screen — more than 600px wide.";
    document.body.style.backgroundColor = "blue";
  }
});