TypeScript: function object parameter with defaults

foo.ts

{
    type All = { foo: string, bar: string };
    type Requireds = Pick<All, 'foo'>;
    type Defaults = Pick<All, 'bar'>;
    // Our function expects all required fields and any overrides for fields with defaults.
    type Args = Requireds & Partial<Defaults>;

    const defaults: Defaults = { bar: 'bar' };
    const fn = (args: Args) => {
        const all: All = { ...defaults, ...args };
    };

    fn({ foo: 'foo', bar: 'bar' }); // overriding default for bar
    fn({ foo: 'foo' }); // using default for bar
    fn({}); // error: foo is required
}