Add query string parameters to a URL in JavaScript (works with browser/node, merges duplicates and works with absolute and relative URLs)

add-query-params-to-url.js

/**
 * Adds query params to existing URLs (inc merging duplicates)
 * @param {string} url - src URL to modify
 * @param {object} params - key/value object of params to add
 * @example
 * // returns /guides?tag=api
 * addQueryParamsToUrl('/guides?tag=hardware', { tag:'api' })
 * @example
 * // returns https://orcascan.com/guides?tag=api
 * addQueryParamsToUrl('https://orcascan.com/guides?tag=hardware', { tag: 'api' })
 * @returns {string} modified URL
 */
function addQueryParamsToUrl(url, params) {

    // if URL is relative, we'll need to add a fake base
    var fakeBase = !url.startsWith('http') ? 'http://fake-base.com' : undefined;
    var modifiedUrl = new URL(url || '', fakeBase);

    // add/update query params
    Object.keys(params).forEach(function(key) {
        if (modifiedUrl.searchParams.has(key)) {
            modifiedUrl.searchParams.set(key, params[key]);
        }
        else {
            modifiedUrl.searchParams.append(key, params[key]);
        }
    });

    // return as string (remove fake base if present)
    return modifiedUrl.toString().replace(fakeBase, '');
}