Simple JavaScript module to manipulate CSS

Question

After posting some of this code on Stack Overflow I was told to have a look here and get some feedback.

I am having fun building a light JavaScript framework and have copied this pattern. It works fine, but I like to have some review on it. The reason that I was directed here is likely the use of CSSStyleDeclaration. I wonder if that is good practice, or just a dirty shortcut.

var $ = (function() {
     function doCSS(prop, val) {
       var isSet = Boolean(val),
         action = CSSStyleDeclaration.prototype.setProperty,
         args = arguments;
       if (isSet) {
         this.each(function(node) {
           action.apply(node.style, args);
         });
         return this;
       } else if (typeof(prop) === 'object') {
         this.each(function(node) {
           Object.keys(prop).forEach(function(property) {
             node.style[property] = prop[property];
           });
         });
         return this;
       } else {
         return this.nodes[0].style[prop];
       }
     }

     // 

     return function(selector, context) {
       var element = context || window.document;
       return {
         selector: selector,
         nodeList: element.querySelectorAll(selector),
         each: function(action) {
           [].forEach.call(this.nodeList, function(item, i) {
             action(item, i);
           });
           return this;
         },
         toString: function() {
           return selector;
         },
         css: function(prop, val) {
           return doCSS.call(this, prop, val);
         }
       };
     };
   }());

   $.extendingFunction = function() {
     alert("I am an extending function");
   };



   $("#string-css").css("background", "seagreen");
   $("#object-css").css({
     background: "olive",
     color: "yellow"
   });

   document.getElementById("extending")
   .addEventListener("click",$.extendingFunction,false);

h1 {
  padding: 5px;
}

<h1 id="string-css">CSS String</h1>

<h1 id="object-css">CSS Object</h1>

<button id="extending">call extended function</button>

Comparing function

As far as the CSSStyleDeclaration goes, I personally am not in favor of it. To be honest, the original was not as dirty as I made by extending it with the option for an object. The following is how I would think it should be.

function doCSS(prop, val) {

    if (typeof (prop) === 'string') {
        this.each(function(node) {
            node.style[prop] = val;
        });
    }

    if (typeof (prop) === 'object') {
        this.each(function(node) {
            Object.keys(prop).forEach(function(property) {
                node.style[property] = prop[property];
            });
        });
    }

    return this;

}