// button '+'
    document.getElementById('add_button').onclick = function() {
        document.getElementById('ok_button').style.display = 'block';
        document.getElementById('add_button').style.display = 'none';
        document.getElementById('input_block').style.display = 'block';
    }

    // button '✔'
    document.getElementById('ok_button').onclick = function() {
        var input_text = document.getElementsByTagName("input")[0].value;
        if (input_text) {
            var new_data = document.createElement('div')
            new_data.className = 'row data'
            new_data.innerHTML = '<div class="cell"><span>' + input_text + '</span><img src="icon_edit.png" class="edit_button"><img src="icon_delete.png" class="remove_button"></div>'
            document.getElementsByClassName('data_table')[0].appendChild(new_data)
            document.getElementById('ok_button').style.display = 'none';
            document.getElementById('add_button').style.display = 'block';
            document.getElementById('input_block').style.display = 'none';
        }
    }

    // function of deleting a row
    function remove_row() {
        this.parentElement.parentElement.remove();
    }

    // row editing function
    function edit_row() {
        row = this.parentElement.parentElement
        row.style.background = '#e9e9e9'

    }

    // update tracking function
    var observer = new MutationObserver(function(mutations) {
        mutations.forEach(function(mutation) {
            if (mutation.addedNodes.length) {
                [btn_edit, btn_remove] = mutation.addedNodes[0].getElementsByTagName('img')
                btn_edit.onclick = edit_row
                btn_remove.onclick = remove_row }});});
    observer.observe(document.getElementsByClassName('data_table')[0], {childList: true})

    // hang action on click
    var el = document.querySelectorAll(".remove_button");
    for (var i = 0; i < el.length; i++) {
        el[i].onclick = remove_row
    }

    var el = document.querySelectorAll(".edit_button");
    for (var i = 0; i < el.length; i++) {
        el[i].onclick = edit_row
    }