With Angular

describe('Youtube api service', () => {
    let service;

    beforeEach(() => {
        angular
            .module('app', [])
            .service('youtubeService', YoutubeService);
        angular.mock.module('app');
    });

    beforeEach(angular.mock.inject((youtubeService) => {
        service = youtubeService;
    }));

    it('should get an ID from long youtube link', () => {
        let link = 'https://www.youtube.com/watch?v=TFeSNOdNtyo';

        let id = service.getId(link);

        expect(id).toBe('TFeSNOdNtyo');
    });

    it('should get an ID from short youtube link', () => {
        let link = 'https://www.youtu.be/TFeSNOdNtyo';

        let id = service.getId(link);

        expect(id).toBe('TFeSNOdNtyo');
    });

    it('should get an ID from same ID', () => {
        let inputID = 'TFeSNOdNtyo';

        let id = service.getId(inputID);

        expect(id).toBe('TFeSNOdNtyo');
    });
});

Without Angular

describe('Youtube api service', () => {

    let service;

    beforeEach(() => {
        service = new YoutubeService();
    });

    it('should get an ID from long youtube link', () => {
        let link = 'https://www.youtube.com/watch?v=TFeSNOdNtyo';

        let id = service.getId(link);

        expect(id).toBe('TFeSNOdNtyo');
    });

    it('should get an ID from short youtube link', () => {
        let link = 'https://www.youtu.be/TFeSNOdNtyo';

        let id = service.getId(link);

        expect(id).toBe('TFeSNOdNtyo');
    });

    it('should get an ID from same ID', () => {
        let inputID = 'TFeSNOdNtyo';

        let id = service.getId(inputID);

        expect(id).toBe('TFeSNOdNtyo');
    });
});