Failed to create URL with base address contained port 80

[Eykhler]

Input

new URL('/part', 'http://servername:80/app');

Result in error: 'URL unable to set base http://servername/app due to http://servername:80/app'

Another problem is that the address must be lowercase due to case-sensitive string comparison, i.e. I can not write

new URL('/part', 'http://SERVERNAME/app');

[lifaon74]

Well, origin (second argument) should not contain any path. This is an error on your side even if some browsers allows it.
Second, domain name are lower-cased, this is normal behavior...

[Eykhler]

Looking at the https://developer.mozilla.org/en-US/docs/Web/API/URL article and Usage example we see that the base part may not be the origin but any base address:

const url = new URL('../cats', 'http://www.example.com/cats');

Casting a domain name to lowercase before creating URL object is not a problem, but again it will be more convenient to ignore case sensitivity when building URLs

var polyfillURL = function() {
  var _URL = global.URL;

  var URL = function(url, base) {
      if (base && (global.location === void 0 || base !== global.location.href)) {
        ....
        try {
           if (baseElement.href.indexOf(base.toLowerCase()) !== 0) throw new Error(baseElement.href);
        } catch (err) {
          throw new Error('URL unable to set base ' + base + ' due to ' + err);
        }
      }