JSFiddle - React, Tailwind, and code Playground
by ham ism
HTML
<div id="test">
<img src="http://placehold.it/300x300" />
</div>
<script src="https://code.jquery.com/jquery-2.1.1.min.js"></script>
CSS
#test{
width:300px
}
JavaScript
var aspectRatio = 9/16;
$('#test img').css('width','100%');
var imgWidth = $('#test img').css('width');
// we have to remove the suffix px in imgWidth otherwise we will get
// a NaN value when we attempt to use that value in arithmetic
// operations
imgWidth = imgWidth.replace('px','');
// above the image width is converted from percentage format(%) to a
// static value for example 300px
var newHeight = imgWidth*aspectRatio;
$('#test img').css('height',newHeight+'px');
console.log(imgWidth+'\n');
console.log(newHeight);