MediaQueryListEvent

var mql = window.matchMedia("(max-width: 600px)");

function screenTest(e) {
  if (e.matches) {
    /* 视口等于或小于 600 像素 */
    para.textContent = "This is a narrow screen — less than 600px wide.";
    document.body.style.backgroundColor = "red";
  } else {
    /* 视口大于 600 像素 */
    para.textContent = "This is a wide screen — more than 600px wide.";
    document.body.style.backgroundColor = "blue";
  }
}

mql.addListener(screenTest);