<p>Lorem Ipsum</p>
<p class="middle">Lorem Ipsum</p>
<p class="middle">Lorem Ipsum</p>
<p>Lorem Ipsum</p>

$('p').filter('.middle'); // Retorna somente o "p" que tiver a classe "middle"

var elementos = document.querySelectorAll('p.middle');

function filter(selElementos, selFiltro) {
    var els = document.querySelectorAll(selElementos);
    var filtro = document.querySelectorAll(selFiltro);
    var saida = [];
    for(var i=0; i<els.length; i++) {
        for(var j=0; j<filtro.length; j++) {
            if(els[i] == filtro[j]) {
                saida.push(els[i]);
            }
        }
    }
    return saida;
}

function filter(selector){
    var aryPMiddle = document.querySelectorAll(selector);
    console.log(aryPMiddle);
    return aryPMiddle;
}

<p id="middle">Lorem Ipsum</p>
<p class="middle">Lorem Ipsum</p>
<p class="middle">Lorem Ipsum</p>
<p>Lorem Ipsum</p>

filter('.middle');

filter('#middle');

var cls = 'middle';
var tag = 'P';

var lista = document.getElementsByTagName(tag);
for (var i = 0; i < lista.length; i++)
{
    if ((' ' + lista[i].className + ' ').indexOf(' ' + cls + ' ') > -1)
    {
        // tratar o elemento com a classe
    }
}