JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>Rum.js</li>
    <li>Yum!</li>
</ul>

CSS

.foo{
    font-size:30px;
    color:red;
}
.foo2{
    font-size:60px;
    color:blue;
}

JavaScript

var rum = function(selector){
    return new rum.fn.init(selector);
};

rum.fn = rum.prototype = {
    
    constructor:rum,
    
    init:function(selector){
        var nodes = document.querySelectorAll(selector);
        for (var i = 0; i < nodes.length; i++) {
            this[i] = nodes[i];
        }
        this.length = nodes.length;
        return this;
    },
    
    rum:'0.1',
    
    addClass:function(classString){
        this.each(function(){
           this.classList.add(classString); 
        }); 
        return this;
    },

    each:function(callback){
        return rum.each(this, callback);
    }   
    
};

rum.each = function(object, callback){
    var name;

    for (name in object) {
        if (callback.call( object[ name ], name, object[ name ] ) === false){
            break;
        }
    }
    return object;
};

rum.fn.init.prototype = rum.fn;

//ok use rum like one uses jQuery
rum('li').addClass('foo2');