Response to

HTML

<span></span>
<span></span>
<span></span>
<p class="test"></p>
<p></p>

JavaScript

var jMoo = new Class({
    Extends: Array,
    initialize: function(selector, context){
        selector = selector || "";
        context = context || document;
        this.push.apply(this, selector.nodeType ? [selector] : context.querySelectorAll(selector));
    },
    first: function(){
    	return this.slice(0,1);
    },
    last: function(){
    	return this.slice(this.length-1,this.length);
    }
});
// Automatically inherits forEach, map, reduce, filter,
// push, pop, slice, splice! Wow...

var jmoo = new jMoo('span, .test');

console.log(jmoo);
console.log(jmoo.first()[0]);
console.log(jmoo.last()[0]);

var len = jmoo.length;
console.log(len);
jmoo.length = len - 1;
console.log(jmoo.length);
console.log("at index " + (len - 1), jmoo[len - 1]); // should be undefined