How to detect DIV's dimension changed?

Question

I\'ve the following sample html, there is a DIV which has 100% width. It contains some elements. While performing windows re-sizing, the inner elements may be re-positioned,

Answer 1

var div = document.getElementById('div');
div.addEventListener('resize', (event) => console.log(event.detail));

function checkResize (mutations) {
    var el = mutations[0].target;
    var w = el.clientWidth;
    var h = el.clientHeight;
    
    var isChange = mutations
      .map((m) => m.oldValue + '')
      .some((prev) => prev.indexOf('width: ' + w + 'px') == -1 || prev.indexOf('height: ' + h + 'px') == -1);

    if (!isChange)
      return;

    var event = new CustomEvent('resize', {detail: {width: w, height: h}});
    el.dispatchEvent(event);
}

var observer = new MutationObserver(checkResize); 
observer.observe(div, {attributes: true, attributeOldValue: true, attributeFilter: ['style']});

#div {width: 100px; border: 1px solid #bbb; resize: both; overflow: hidden;}

DIV