Stick the url with parameters. Covered by unit tests on Jest

getUrlWithParams.js

const paramsReducer = function paramsReducer(acc, param) {
    const splittedParam = param.split('=');
    const key = splittedParam[0];
    const value = splittedParam[1];

    return Object.assign({}, acc, { [key]: value });
};

/**
 * Returns the url with the pasted parameters
 * @param {string} url Url to which you want to stick the params
 * @param {Object} params Parameters that are stuck to the given url
 * @returns {string} Url with the pasted parameters
 */
export default function getUrlWithParams(url, params = {}) {
    if (typeof url !== 'string') {
        throw new Error('url should be a String');
    }

    const separatorIndex = url.indexOf('?');

    const defaultPath = (separatorIndex === -1)
        ? url
        : url.substring(0, separatorIndex);

    const defaultParams = (separatorIndex === -1)
        ? {}
        : url.substring(separatorIndex + 1).split('&').reduce(paramsReducer, {});

    const joinedParams = Object.assign({}, defaultParams, params);

    const clearParamsKeys = Object.keys(joinedParams).filter((key) => {
        const value = joinedParams[key];
        return (typeof value !== 'undefined' && value !== null && value !== '');
    });

    const urlParams = clearParamsKeys.map((key) => `${key}=${joinedParams[key]}`);

    const separator = urlParams.length !== 0 ? '?' : '';

    const resultUrl = `${defaultPath}${separator}${urlParams.join('&')}`;

    return resultUrl;
}

getUrlWithParams.test.js

import getUrlWithParams from './getUrlWithParams';

describe('getUrlWithParams()', () => {
    test('should return an empty string if the url is not passed', () => {
        expect(getUrlWithParams('')).toEqual('');
    });

    test('should return an unchanged url if parameters are not passed', () => {
        expect(getUrlWithParams('https://site.com')).toEqual('https://site.com');

        expect(getUrlWithParams('https://site.com?')).toEqual('https://site.com');

        expect(getUrlWithParams('https://site.com?defaultParam1=1')).toEqual('https://site.com?defaultParam1=1');

        expect(getUrlWithParams('https://site.com', {})).toEqual('https://site.com');

        expect(getUrlWithParams('https://site.com?', {})).toEqual('https://site.com');

        expect(getUrlWithParams('https://site.com?defaultParam1=1', {})).toEqual('https://site.com?defaultParam1=1');

        expect(getUrlWithParams(
            'https://site.com?defaultParam1=1&defaultParam2=2', {})
        ).toEqual('https://site.com?defaultParam1=1&defaultParam2=2');

        expect(
            getUrlWithParams('https://site.com', {
                param1: undefined,
                param2: null,
                param3: ''
            })
        ).toEqual('https://site.com');

        expect(
            getUrlWithParams('https://site.com?defaultParam=1', {
                param1: undefined,
                param2: null,
                param3: ''
            })
        ).toEqual('https://site.com?defaultParam=1');
    });

    test('should return the url that is linked with the parameters', () => {
        expect(
            getUrlWithParams('https://site.com', {
                param1: undefined,
                param2: null,
                param3: '',
                param4: 'value4',
                param5: 'value5'
            })
        ).toEqual('https://site.com?param4=value4&param5=value5');

        expect(
            getUrlWithParams('https://site.com?defaultParam1=1', {
                param1: undefined,
                param2: null,
                param3: '',
                param4: 'value4',
                param5: 'value5'
            })
        ).toEqual('https://site.com?defaultParam1=1&param4=value4&param5=value5');

        expect(
            getUrlWithParams('https://site.com?defaultParam1=1&defaultParam2=2', {
                param1: undefined,
                param2: null,
                param3: '',
                param4: 'value4',
                param5: 'value5'
            })
        ).toEqual('https://site.com?defaultParam1=1&defaultParam2=2&param4=value4&param5=value5');
    });
});