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"]