How to write unit test case for the given code

Question

VacationFRP = [{key: 'test', value: '123456'}, {key: 'test1', value: '654321'}];
links = [{id: 'test', showTab: false}, {id: 'test1', showTab: false}];
VacationFRP.forEach(frp => {
    from(!!(frp.value)).subscribe(access => {
        const index = this.links.findIndex(item => {
            return item.id === frp.key;
        });
        this.links[index].showTab = access;
    });
});

I am facing issue to write unit test case for the above given code