A convenience wrapper around querySelector/querySelectorAll, with a transform function to turn NodeLists into arrays.

query.js

/* jshint -W033, -W097 */
/* globals document, Element */
(function() {

    "use strict";

    function query(es, s) {
        return _q(es, s, "querySelector")
    }

    function queryAll(es, s) {
        return _q(es, s, "querySelectorAll")
    }

    /**
     * Create an array from enumerable elements in a node list.
     * Useful if you want to map it, slice it, etc.
     * @param  {NodeList} nodeList A selection of nodes, such as from document.querySelectorAll.
     * @return {Array}
     */
    function nodeArray(nodeList) {
        return Array.prototype.slice.call(nodeList)
    }

    /**
     * Function internals for querySelector/querySelectorAll.
     * 1. Exit if qs/qsA not supported.
     * 2. If multiple arguments received, try selecting descendants
     *    of 1st parameter (either an HTML element (3.) or string (4.) will work),
     *    assume second argument is a string in that case.
     * 5. If no results yet and `es` is a string, query it.
     *
     * @param  {Element|String} es      Either an HTML element or a selector
     * @param  {String} s               Selector
     * @param  {String} queryFunction   "querySelector"|"querySelectorAll"
     * @return {*}
     */
    function _q(es, s, queryFunction) {
        var root = document
        var el
        var r

        if (typeof document[queryFunction] === "undefined") return // 1.

        if (arguments.length > 1 && typeof s === "string") { // 2.
            if (es instanceof Element) { root = es } // 3.
            if (typeof es === "string") { // 4.
                el = document.querySelector(es)
                root = el ? el : root
            }
            r = root[queryFunction](s)
        }

        if (!r && typeof es === "string") { // 5.
            r = root[queryFunction](es)
        }

        if (r) { return r.length ? nodeArray(r) : r }
    }

    // export to global scope
    window.query = query
    window.queryAll = queryAll
    window.nodeArray = nodeArray

}());