Getting part of a domain name by host name. Covered by unit tests on Jest

getDomainByHostname.js

/**
 * Returns domain name's part by given hostname and level
 * @param {string} hostname
 * @param {Number} level
 * @returns {string} domain name's part
 */
export default function getDomainByHostname(hostname = "", level = 0) {
    return hostname.split('.').slice(-level).join('.');
}

getDomainByHostname.test.js

import getDomainByHostname from './getDomainByHostname';

describe('getDomainByHostname()', () => {
    test('should return empty string if the hostname has not been passed', () => {
        expect(getDomainByHostname()).toEqual('');
        expect(getDomainByHostname('')).toEqual('');
        expect(getDomainByHostname('', 1)).toEqual('');
        expect(getDomainByHostname('', 2)).toEqual('');
    });

    test('should return first level domain', () => {
        expect(getDomainByHostname('com', 1)).toEqual('com');
        expect(getDomainByHostname('site.com', 1)).toEqual('com');
        expect(getDomainByHostname('third.site.com', 1)).toEqual('com');
        expect(getDomainByHostname('fouth.third.site.com', 1)).toEqual('com');
    });

    test('should return seconde level domain', () => {        
        expect(getDomainByHostname('site.com')).toEqual('site.com');
        expect(getDomainByHostname('site.com', 2)).toEqual('site.com');
        expect(getDomainByHostname('third.site.com', 2)).toEqual('site.com');
        expect(getDomainByHostname('fouth.third.site.com', 2)).toEqual('site.com');
    });

    test('should return third level domain', () => {
        expect(getDomainByHostname('third.site.com')).toEqual('third.site.com');
        expect(getDomainByHostname('third.site.com', 3)).toEqual('third.site.com');
        expect(getDomainByHostname('fouth.third.site.com', 3)).toEqual('third.site.com');
    });
});