Knockout extender: filter

filter.js

ko.extenders.filter = function (target, pattern) {
    var result = ko.computed({
        read: target,
        write: function (value) {
            if (typeof value !== "string") {
                target(value);
                return;
            }
            var current = target();
            var filtered = value.replace(pattern, "");
            if (filtered !== current) {
                target(filtered);
            } else if (value !== current) {
                target.notifySubscribers(filtered);
            }
        }
    }).extend({ notify: "always" });
    result(target());
    return result;
};

var x = ko.observable().extend({ filter: /[aeiou]/gi });
x("hello");
console.log(x());
x("WORLD");
console.log(x());

output.txt

hll
WRLD