JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo">Hello</div>

CSS

.a {
    background: green;
}

.b {
    color: white;
}

.c {
  text-align: center;
}

JavaScript

function addClass(element, myClass) {
    element.className += ' ' + myClass; 
}

function addClasses(element, classArr) {
    for (var i = 0; i < classArr.length; i++) {
        addClass(element, classArr[i]);
    }
}

var foo = document.getElementById('foo');

var classes = [];

addClasses(foo,);

addClass(foo, 'a');
addClass(foo, 'b');
addClass(foo, 'c');