counter_model.ts

type CounterParams = {
    value?: number,
    status?: Status,
};
export class Counter extends Record({ value: 0, status: OK }) {
    value: number;
    status: Status;

    constructor(params?: CounterParams) {
        params ? super(params) : super();
    }

    with(values: CounterParams) {
        return this.merge(values) as this;
    }
}

Note that the CounterParams type has all fields as optional. This is so we can construct or update a Counter with a subset of the fields, relying on the defaults passed to Record to supply the rest.