x.js is my first attempt to build jQuery-like micro library. It has most of the functionality you use every day - selectors, events, traversal, add/remove/toggle class, get/set attributes and handles most common cases.

x.js

;
(function (window) {

    'use strict';

    var xBuild = (function () {

        var xGlobals = {
            newElementRegex: /<(\w+)\s?\/>/,
        };

        var x = function (_expression, _context) {
            return new x.fn.init(_expression, _context);
        };

        //--------------------------------------------------------------
        //  Extending X with util functions
        //--------------------------------------------------------------
        x.addHelper = function (_name, _fn) {
            x[_name] = _fn;
        };

        //--------------------------------------------------------------
        //  Simple get request helper, just an example how to add helpers
        //--------------------------------------------------------------
        x.addHelper('get', function (_url, _onDone, _onFail) {
            var request = new XMLHttpRequest();
            request.open('GET', _url, true);

            request.onload = function () {
                if (request.status >= 200 && request.status < 400) {
                    _onDone(request.responseText);
                } else {
                    _onFail(request.responseText)
                }
            };

            request.onerror = _onFail;

            request.send();
        });

        //--------------------------------------------------------------
        //  Build prop is used internally 
        //--------------------------------------------------------------

        x.addHelper('buildProp', function (_prop, _val) {
            var props = {};
            if (_val !== undefined) {
                props[_prop] = _val;
            } else {
                props = _prop;
            }

            return props;
        });

        //--------------------------------------------------------------
        //  Core
        //--------------------------------------------------------------
        x.fn = x.prototype = {
            init: function (_expression, _context) {

                if (typeof _expression === 'function') {
                    document.addEventListener('DOMContentLoaded', _expression);
                } else {

                    this.elements = [];
                    var newElement = xGlobals.newElementRegex.exec(_expression);
                    if (newElement) {
                        this.elements.push(document.createElement(newElement[1]));
                    } else if (_expression.tagName) {
                        this.elements.push(_expression);
                    } else if (_expression.constructor === Array) {
                        this.elements = _expression;
                    } else {
                        var nodes = (_context == null ? document : _context).querySelectorAll(_expression);
                        for (var i = 0, j = nodes.length; i < j; i++) {
                            this.elements.push(nodes[i]);
                        }
                    }
                    return this;
                }
            },

            el: function () {
                return this.first().elements[0];
            },

            forEach: function (_fn) {
                this.elements.forEach(_fn);
                return this;
            },

            on: function (_handler, _fn) {
                return this.forEach(function (el) {
                    el.addEventListener(_handler, _fn);
                });
            },

            off: function (_handler, _fn) {
                return this.forEach(function (el) {
                    el.removeEventListener(_handler, _fn);
                });
            },

            trigger: function (_event, _data) {
                return this.forEach(function (elem) {
                    elem.dispatchEvent(new CustomEvent(_event, {detail: _data || {}}))
                });
            },

            addClass: function (_className) {
                return this.forEach(function (elem) {
                    elem.classList.add(_className);
                });
            },

            removeClass: function (_className) {
                return this.forEach(function (elem) {
                    elem.classList.remove(_className);
                });
            },

            toggleClass: function (_className) {
                return this.forEach(function (elem) {
                    elem.classList.toggle(_className);
                });
            },

            hasClass: function (_className) {
                var hasClass = false;
                this.forEach(function (elem) {
                    if (elem.classList.contains(_className)) {
                        hasClass = true;
                    }
                });

                return hasClass;
            },

            text: function (_newText) {

                if (_newText === undefined) {
                    return this.el().textContent;
                }

                return this.forEach(function (elem) {
                    elem.innerText = _newText;
                });
            },

            html: function (_newHtml) {

                if (_newHtml === undefined) {
                    return this.el().innerHTML;
                }

                return this.forEach(function (elem) {
                    elem.innerHTML = _newHtml;
                });
            },

            find: function (_selector) {
                var elements = [];
                this.forEach(function (elem) {
                    var finds = elem.querySelectorAll(_selector);
                    if (finds.length) {
                        for (var i = 0, j = finds.length; i < j; i++) {
                            elements.push(finds[i]);
                        }
                    }
                });
                return x(elements);
            },

            empty: function () {
                return this.html('');
            },

            remove: function () {
                return this.forEach(function (elem) {
                    elem.parentNode.removeChild(elem);
                });
            },

            //Need to implement those;
            append: function (_elem) {

            },

            prepend: function (_elem) {

            },

            before: function (_elem) {

            },

            after: function (_elem) {

            },

            clone: function () {
                return this.el().cloneNode(true);
            },

            attr: function (_attrName, _attrVal) {
                if (_attrVal === undefined) {
                    return this.el().getAttribute(_attrName);
                }

                var attrObj = x.buildProp(_attrName, _attrVal);

                this.forEach(function (elem) {
                    for (var attr in attrObj) {
                        if (attrObj.hasOwnProperty(attr)) {
                            elem.setAttribute(attr, attrObj[attr]);
                        }
                    }
                });

                return this;
            },

            css: function (_cssObj, _cssVal) {

                var stylesObj = x.buildProp(_cssObj, _cssVal);

                return this.forEach(function (elem) {
                    for (var rule in stylesObj) {
                        if (stylesObj.hasOwnProperty(rule)) {
                            elem.style[rule] = stylesObj[rule];
                        }
                    }
                });
            },

            parent: function () {
                return x(this.el().parentNode);
            },

            first: function () {
                return this.eq(0);
            },

            last: function () {
                return this.eq(this.elements.length - 1);
            },

            eq: function (_index) {
                return x(this.elements[_index]);
            }

        };


        //--------------------------------------------------------------
        //  Expose to global
        //--------------------------------------------------------------
        x.fn.init.prototype = x.fn;
        window.x = x;

    })();

})(window, undefined);