Edit in JSFiddle

var testEl = document.getElementById('test');
console.log('div width', testEl.clientWidth);
console.log('div height', testEl.clientHeight);

console.log('body width', document.body.clientWidth);
console.log('body height', document.body.clientHeight);

<div id="test"></div>

#test {
  width: 100%;
  height: 100%;
}