first screen max height main-header

JS
let mainHeader = document.getElementById('main-header');
setMainHeaderHeight();
  window.onresize = function () {
    setMainHeaderHeight();
  }
  function setMainHeaderHeight() {
    let height;
    if (typeof document.height !== 'undefined') {
      height = document.height // For webkit browsers
    } else {
      height = document.documentElement.clientHeight;
    }
    (height > 1200) && (height = 1200);
    mainHeader.style.minHeight = `${height}px`;
  }