JSFiddle - React, Tailwind, and code Playground
HTML
<script src="http://underscorejs.org/underscore.js"></script>
JavaScript
var a = ['js-email'],
b = ['js-email', 'form-group'],
c = _.difference(a, b),
d = _.difference(b, a);
console.log('diff:', c); // diff: []
console.log('diff:', d); // diff: ["form-group"]