Conditional Validation with onlyIf parameter

Sometimes you will have a viewmodel property that is only required in certain circumstances. You can use the onlyIf parameter, which accepts a function, to specify when that field is required. The required validator will only be applied if the onlyIf function returns true.

For example, consider an address block that requires a postal code unless a "noPostalCode" field is set to true.

    self.postalCode.extend({
        required: {
            message: "Postal Code is required",
            onlyIf: function () { return (self.noPostalCode() === false); }
        }
    });

See another example here on jsFiddle.

Conditional validation ('onlyIf') can be used in combination with all other KO validation rules (e.g. max, min, etc.)