function eventlistener() {
    var bns = document.getElementsByTagName('button');
    for (var i = 0; i < bns.length; i++) {
        bns[i].addEventListener('click', function() {
        bns[i].value(bns[i].getAttribute("data-text").toString);
    },false);
    }
}

window.addEventListener("load",function() {
    eventlistener();
});