Add path to url

appendPath.js

// https://stackoverflow.com/questions/27846195/how-can-i-add-the-current-pathname-to-a-href-with-a-different-url/74437330#74437330
// https://jsfiddle.net/t41z39fv/

const cases = [
  ["http://a.b", "a", "http://a.b/a"],
  ["http://a.b", "/a", "http://a.b/a"],
  ["http://a.b/", "a", "http://a.b/a"],
  ["http://a.b/a", "a", "http://a.b/a/a"],
  ["http://a.b/a/", "a", "http://a.b/a/a"],
  ["http://a.b", "a/a", "http://a.b/a/a"],
  ["http://a.b/", "a/a", "http://a.b/a/a"],
  ["http://a.b/a", "a/a", "http://a.b/a/a/a"],
  ["http://a.b/a/", "a/a", "http://a.b/a/a/a"],
  ["http://a.b/a/a", "a/a", "http://a.b/a/a/a/a"],
  ["http://a.b/a/a/", "a/a", "http://a.b/a/a/a/a"],
  ["http://a.b?", "/a", "http://a.b/a?"],
  ["http://a.b/?", "a", "http://a.b/a?"],
  ["http://a.b?a=b", "/a", "http://a.b/a?a=b"],
  ["http://a.b/?a=b", "a", "http://a.b/a?a=b"],
  ["http://a.b", "a b", "http://a.b/a%20b"],
  ["http://a.b", "/a b", "http://a.b/a%20b"],
]

const appendPath = (href, path) => {
  if (!path) return href;
  const url = new URL(href);
	url.pathname += `${url.pathname.endsWith('/') ? "" : "/"}${path.startsWith('/') ? path.slice(1) : path}`
	return url.href;
}

const fail = cases.find(([url, path, expected]) => {
  const actual = appendPath(url, path);
  if (actual !== expected) console.log(actual);
  return actual !== expected;
})

console.log(fail ?? "success");