Setting the height of a DIV dynamically

Try this simple, specific function:

function resizeElementHeight(element) {
  var height = 0;
  var body = window.document.body;
  if (window.innerHeight) {
      height = window.innerHeight;
  } else if (body.parentElement.clientHeight) {
      height = body.parentElement.clientHeight;
  } else if (body && body.clientHeight) {
      height = body.clientHeight;
  }
  element.style.height = ((height - element.offsetTop) + "px");
}

It does not depend on the current distance from the top of the body being specified (in case your 300px changes).

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EDIT: By the way, you would want to call this on that div every time the user changed the browser’s size, so you would need to wire up the event handler for that, of course.