JSFiddle - React, Tailwind, and code Playground

HTML

<div id="A"></div>
<div id="B"></div>

CSS

#A{
    background-color: red;
}

#B{
    background-color: blue;
}
#A,#B{
    width: 100px;
    height: 100px;
}

JavaScript

var animations = [];
animations.push($('#A').animate({
    width: 200
}, 1000));
animations.push($('#B').animate({
    height: 200
}, 1000));

$.when.apply($, animations).then(function() {
    alert('a');
});