The W lib (Mini JQuery with some underscore)

W.js

// Wilib: lightweigth JQuery clone with no ie support
// $ is replace by W

window.W = function(selector) {
    if (selector[0] == '<' && selector[selector.length - 1] == '>') {
        var elem = selector.replace('>', '').replace('<', '');
        var node = document.createElement(elem);
        wObject(node);
        return node;
    }
    var elems = document.querySelectorAll(selector);
    return wArray(elems);
}

W.select = function(data, props) {
    var newArray = [];
    data.forEach(function(e) {
        var newObj = {};
        if (typeof(props) === "string")
            props = [props];
        props.forEach(function(prop) {
            if (e[prop] !== undefined)
                newObj[prop] = e[prop];
        });
        newArray.push(newObj);
    });
    return newArray;
};

W.where = function(data, predicate) {
    var newArray = [];
    data.forEach(function(e) {
        var add = true;
        for (pred in predicate) {
            if (predicate[pred] != e[pred]) {
                add = false;
                break;
            }
        }
        if (add)
            newArray.push(e);
    });
    return newArray;
};

W.postJSON = function(url, data) {
    return new Promise(function(resolve, reject) {
        var req = new XMLHttpRequest();
        req.open('POST', url, true);
        req.onreadystatechange = function() {
            if (req.readyState == 4) {
                if (req.status == 200)
                    resolve(JSON.parse(req.responseText));
                else
                    throw new Error(req.statusText);
            }
        };
        req.onerror = function() {
            throw new Error("network error");
        };
        req.send(data);
    });
};

W.getJSON = function(url, data) {
    return new Promise(function(resolve, reject) {
        var req = new XMLHttpRequest();
        req.open('GET', url, true);
        req.onreadystatechange = function() {
            if (req.readyState == 4) {
                if (req.status == 200)
                    resolve(JSON.parse(req.responseText));
                else
                    throw new Error(req.statusText);
            }
        };
        req.onerror = function() {
            throw new Error("network error");
        };
        req.send(data);
    });
};

var wObject = function(node) {
    node.on = function(events, callback) {
        var eventsArray = events.split(',');
        for (var e in eventsArray)
            this['on' + eventsArray[e]] = function(event) {
                event.origin = this;
                event.target = wObject(event.target);
                callback(event);
            }
        return this;
    };
    node.css = function(styles) {
        for (var prop in styles)
            this.style[prop] = styles[prop];
        return this;
    };
    node.find = function(selector) {
        var elems = this.querySelectorAll(selector);
        return wArray(elems);
    }
}

var wArray = function(that) {

    //Add function on node
    for (var i = 0; i < that.length; i++) {
        wObject(that[i]);
    }

    //distribute function on NodesArray
    that.on = function(event, callback) {
        for (var i = 0; i < this.length; i++)
            this[i].on(event, callback);
        return this;
    };

    that.css = function(styles) {
        for (var i = 0; i < this.length; i++)
            this[i].css(styles);
        return this;
    };

    that.data = function(data, set) {
        if (set !== undefined) {
            for (var i = 0; i < this.length; i++)
                this[i].dataset[data] = set;
            return this;
        } else {
            return this[0].dataset[data];
        }
    };

    //classic forEach
    that.forEach = function(callback) {
        for (var i = 0; i < this.length; i++) {
            callback(this[i]);
        }
        return this;
    };

    return that;
};