JSFiddle - React, Tailwind, and code Playground
HTML
<div class="left"></div>
<div class="right"></div>
CSS
.left {
width:100px;
background:red;
height: 200px;
float:left;
-webkit-transition: all .4s;
-moz-transition: all .4s;
}
.right{
background:blue;
width: auto;
height:200px;
}
JavaScript
var size = ['100px', '400px'];
var i = 0;
$('.left').click(function(){
i = (i + 1 > 1)? 0: ++i;
$(this).css('width', size[i]);
});