Knockout extender: coalesce

coalesce.js

ko.extenders.coalesce = function (target, defaultValue) {
    var result = ko.computed({
        read: function () {
            return target() === undefined ? ko.unwrap(defaultValue) : target();
        },
        write: function (value) {
            target(value);
        }
    });
    result.defaultValue = ko.computed(function () {
        return ko.unwrap(defaultValue);
    });
    return result;
};

var x = ko.observable().extend({ coalesce: 1 });
console.log(x());
x(2);
console.log(x());
console.log(x.defaultValue());
x(undefined);
console.log(x());
var y = ko.observable(3);
var z = ko.observable().extend({ coalesce: y });
console.log(z());
y(4);
console.log(z());

output.txt

1
2
1
1
3
4