window.onload = setDiv;
 
function setDiv() {
  var wh = getWindowHeight(); // Window Height
  var d = document.getElementById('divID') // Get div element
  var dh = d.offsetHeight // div height
  d.style.height = wh + 'px'; // Set div height to window height
}
 
 
 
 
function getWindowHeight() {
  var windowHeight = 0;
 
  if (typeof(window.innerHeight) == 'number')
    windowHeight = window.innerHeight;
 
  else {
 
    if (document.documentElement && document.documentElement.clientHeight)
      windowHeight = document.documentElement.clientHeight;
 
    else {
      if (document.body && document.body.clientHeight)
        windowHeight = document.body.clientHeight; }; };
 
  return windowHeight;
};

d.style.minHeight = wh + 'px'; // Set minimum div height to window height

if (dh < wh)
  d.style.height = wh + 'px'; // Set div height to window height only if div height is less than window height;