JSFiddle - React, Tailwind, and code Playground

HTML

<div style="background-color:#222; width:100px; height:100px;"class="test1 test2 test3" onclick="toggleClass(this,'foo')"></div>

CSS

.foo {
    background-color:forestgreen !important;
}

JavaScript

function toggleClass(e, c) {
	var classes = e.className.split(' ');
	var match = false;
	for(var i=0; i<classes.length; i++) {
		if(classes[i] === c) {
			match = true;
			classes.splice(i,1);
			break;
		}
	}
	if(!match) classes.push(c);
	e.className = classes.join(' ');
}