Как получить style width при его изменении?

let initHeight = 10 
let currentHeight = initHeight 
let slider = document.getElementById('slider') 
let button = document.getElementById('button') 
 
slider.addEventListener("transitionend", showMessage, false); 
button.addEventListener('click', setHeight) 
 
slider.style.height = `${initHeight}px` 
 
function setHeight() { 
  slider.style.height = `${++currentHeight}px` 
} 
 
function showMessage () { 
console.log(slider.style.height) 
}

#slider{ 
  width: 100%; 
  height: 100px; 
  background: #8BC34A; 
  transition: height 0.001s; 
}

<div id="slider"></div> 
<button id="button">Увеличить высоту</button>