String empty to null custom binding

Question

Based on this answer Knockout.js: time input format and value restriction, i'm trying to create a custom binding that sets observable to null if the value is string empty, here is the code that's not working, Ip observable is always null on model

ko.bindingHandlers.stringEmptyNull = {
        init: function (element, valueAccessor, allBindingsAccessor) {
            var underlyingObservable = valueAccessor();
            var interceptor = ko.dependentObservable({
                read: underlyingObservable,
                write: function (value) {
                    if (value != null && value.trim() == '')
                        underlyingObservable();
                }
            });
            ko.bindingHandlers.value.init(element, function () { return interceptor }, allBindingsAccessor);
        },
        update: ko.bindingHandlers.value.update
    };

input:

<input type="text" data-bind="stringEmptyNull: Ip" />

Model:

var Model = function () {
        this.Ip = ko.observable()
        ko.applyBindings(this, $myForm[0]);
    };

    instance = new Model();

Answer 1

Your problem is in your interceptor.

In your write function you need to set the underlyingObservable to null when the value is an empty string (calling only underlyingObservable(); only gets its value but does not set its value to null) and you also need to pass value to underlyingObservable in every other case:

var interceptor = ko.dependentObservable({
    read: underlyingObservable,
    write: function (value) {
        if (value != null && value.trim() == '')
            underlyingObservable(null);
        else
            underlyingObservable(value);
    }
});

Demo JSFiddle. (Open your browser's console then type something in the textbox and then clear the textbox)