JSFiddle - React, Tailwind, and code Playground

HTML

<div class="col-sm-8 mainContent">
    Hello
    <br /><br /><br /><br /><br /><br /><br /><br /><br />
    <br /><br /><br /><br /><br /><br />
</div>
<div class="col-sm-4 sidebar">
    Sidebar
</div>

CSS

.mainContent {
    float:left;
    background:#999;
}
.sidebar {
    float:left;
    background:#e9e9e9;
}

JavaScript

var sideHeight = $('.mainContent').css('height');
$('.sidebar').css('height', sideHeight);