<a class="action-menu-item"
        data-bind="attr: {href: $action().href},
                   text: $action().label,
                   click: $col.getActionHandler($action())"
        data-repeat-index="0"
        href="http://.../index.php/admin/customer/index/edit/id/1/">Edit</a>

$actions['alert'] = [
    'label' => __('Alert'),
    'callback' => 'function() {alert("Action!")}'
];

"alert": {
  "label": "Alert",
  "callback": "function() {alert("Action!")}"
}

getActionHandler: function (action) {
    var index = action.index,
        rowIndex = action.rowIndex;

    if (this.isHandlerRequired(index, rowIndex)) {
        return this.applyAction.bind(this, index, rowIndex);
    }
},

applyAction: function (actionIndex, rowIndex) {
    debugger
    var action = this.getAction(rowIndex, actionIndex),
        callback = this._getCallback(action);

    action.confirm ?
        this._confirm(action, callback) :
        callback();

    return this;
},

_getCallback: function (action) {
    var args = [action.index, action.recordId, action],
        callback = action.callback;

    if (utils.isObject(callback)) {
        args.unshift(callback.target);

        callback = registry.async(callback.provider);
    } else if (_.isArray(callback)) {
        return this._getCallbacks(action);
    } else if (!_.isFunction(callback)) {
        callback = this.defaultCallback.bind(this);
    }

    return function () {
        callback.apply(callback, args);
    };
},