Knockout - Custom bindings - pass variable from init to update

First time using knockout.js for me.

I have the following binding:

ko.bindingHandlers.dateselect = {
    init: function (element) {

        var picker = new Pikaday({
            field: element,
        });

    },
    update: function (element) {

        var current = picker.getDate();

    }
};

In the init method, I initialise the datepicker and I'm trying to get the date in the update method, as shown above.

Problem being is I'm getting the following error:

picker is not defined

Is there any way to access the picker instance inside the update method?

You can do this by setting up helpers, which are stored to the element.

E.g.

function SomeBindingHelper(element, valueAccessor, allBindings, viewModel, bindingContext) {
    var sharedVariable = "Banana";

    // Call on init.
    this.init = function () {
        console.log("init", sharedVariable);
    }

    // Call on update.
    this.update = function () {
        console.log("update", sharedVariable);
    }
}

ko.bindingHandlers.someBinding = {
    init: function (element, valueAccessor, allBindings, viewModel, bindingContext) {
       var bindingHelper = new SomeBindingHelper(element, valueAccessor, allBindings, viewModel, bindingContext);
       bindingHelper.init();
       ko.utils.domData.set(element, "someBindingHelper", bindingHelper);
    },
    update: function (element, valueAccessor, allBindings, viewModel, bindingContext) {
       var bindingHelper = ko.utils.domData.get(element, "someBindingHelper");
       bindingHelper.update();
    }
}