Knockout extender: numeric

numeric.js

function log(value) {
    console.log(typeof value + ": " + value);
}

ko.extenders.numeric1 = function (target) {
    return ko.computed({
        read: target,
        write: function (value) {
            var number = parseFloat(value);
            target(isNaN(number) ? value : number);
        }
    });
};

var x = ko.observable().extend({ numeric1: {} });
x(1);
log(x());
x("2");
log(x());
x("three");
log(x());

ko.extenders.numeric2 = function (target) {
    target.value = ko.computed(function () {
        return parseFloat(target());
    });
    return target;
};

var y = ko.observable().extend({ numeric2: {} });
y("4");
log(y());
log(y.value());
y("five");
log(y());
log(y.value());

output.txt

number: 1
number: 2
string: three
string: 4
number: 4
string: five
number: NaN