JSFiddle - React, Tailwind, and code Playground

HTML

<div id='box'></div>
<div id='plus' onClick='resize()'>add height</div>

JavaScript

function resize() {
    //get element

    var height = document.getElementById('box').style.height;

    //transform element from string to integer

    height = height.replace("px", "");
    height =+height;

    //change height
    var n = height;
    height = n + 300 + 'px';
    document.getElementById('box').style.height = height
}