Simpe element querying by class/id

query.js

(function (global) {

    function query(selector) {
        if (!selector) return [];
        // format selector (remove double spaces, convert to lower case), convert into array
        selector = selector.toLowerCase().trim().replace(/ +/g, ' ').split(' ');
        return _query(selector, document.body, []);
    }

    function _query(selector, root, result) {

        if (selector.length > 0) {

            var curSelector = selector[0];
            var curResult = [];
            var i = 0;

            if (curSelector.charAt(0) === '.') {
                var className = curSelector.substr(1);

                for (i = 0; i < root.children.length; i++) {
                    var classAttribute = root.children[i].getAttribute('class');

                    if (classAttribute && classAttribute.toLowerCase().split(' ').indexOf(className) !== -1 /* class matched */) {
                        if (selector.length === 1) {
                            // no more selectors
                            result.push(root.children[i]);
                        } else {
                            _query(selector.slice(1), root.children[i], result);
                        }
                    } else {
                        _query(selector, root.children[i], result);
                    }

                }
            } else if (curSelector.charAt(0) === '#') {

                var idName = curSelector.substr(1);

                for (i = 0; i < root.children.length; i++) {
                    var idAttribute = root.children[i].getAttribute('id');

                    if (idAttribute && idAttribute.toLowerCase() === idName /* id matched */) {
                        if (selector.length === 1) {
                            // no more selectors
                            result.push(root.children[i]);
                        } else {
                            _query(selector.slice(1), root.children[i], result);
                        }
                    } else {
                        _query(selector, root.children[i], result);
                    }

                }

            }

        }

        return result;
    }

    global.query = query;

})(window);

Usage:

query(".class1");
query("#id1 .class2")
query(".class1 .class2 #id1")