JSFiddle - React, Tailwind, and code Playground

HTML

<div id="dornierenvoj">
    <img src="http://img269.imageshack.us/img269/2528/asdasdasd2.jpg"/>
</div>

JavaScript

var k = 761;
var height = $('#dornierenvoj img').css('height').replace('px', '');
alert(height - k);