The url.resolve() method resolves a target URL relative to a base URL in a manner similar to that of a web browser resolving an anchor tag.

const url = require('node:url');
url.resolve('/one/two/three', 'four');         // '/one/two/four'
url.resolve('http://example.com/', '/one');    // 'http://example.com/one'
url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'

To achieve the same result using the WHATWG URL API:

function resolve(from, to) {
  const resolvedUrl = new URL(to, new URL(from, 'resolve://'));
  if (resolvedUrl.protocol === 'resolve:') {
    // `from` is a relative URL.
    const { pathname, search, hash } = resolvedUrl;
    return pathname + search + hash;
  }
  return resolvedUrl.toString();
}

resolve('/one/two/three', 'four');         // '/one/two/four'
resolve('http://example.com/', '/one');    // 'http://example.com/one'
resolve('http://example.com/one', '/two'); // 'http://example.com/two'