JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btn">push!</button>
<div class="cont">
    <div class="first"></div>
    <div class="second"></div>
</div>

CSS

.cont>div {
    float: left;
    height: 300px;
    transition: width 1s ease-out 0.5s;
}
.first { width: 20%; background-color: blue; }
.second { width: 80%; background-color: red; }
.cont.resized>div { width: 50%; }

JavaScript

$('#btn').on('click', function(){
    $('.cont').toggleClass('resized')
})