JSFiddle - React, Tailwind, and code Playground

HTML

<div class="Square15px" onmouseover="changeColor('boxid')">xx</div>

JavaScript

$(function() {
    var boxid = 'box1';
    var AquaStr1 = '<div class="AquaSquare15px" onmouseover="changeColor(\'';
    var AquaStr2 = '\')">yy</div>';
    
    var AquaString = AquaStr1.concat(boxid, AquaStr2);
    $('body').append(AquaString);
});