What determines if a URL needs a trailing slash?

Trailing slash = treated as a directory by the server.
No trailing slash = treated as a file or resource.

Historical/Old-school view

https://example.com/foo/ → folder → server looks for /foo/index.html.
https://example.com/foo → file → server looks for /foo.

Browsers still respect this: a URL without a trailing slash that’s a directory usually triggers a 301 redirect to add /.

Use server/SPA conventions:
Routes ending in IDs, hashes, or single resources → “file-like” → no slash.
Routes representing collections, folders, or sections → “directory-like” → usually slash.
If you want to be safe, check what the server considers canonical: visit the URL → see if it redirects → copy that URL.

service layer

use this!

const API_ROOT = "http://xxx.com/api/";

'http://xxx.com/api/?filter=big';

works okay, works.
The trailing slash signals “this is a base URL you append paths to.”

---

double slashes don't work

https://mokat6.github.io/notes-docusaurus//clean-code/intro/

error not found 404.

some servers allow it though.

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you could create a helper function

function joinUrl(base, segment) {
  return `${base.replace(/\/$/, "")}/${segment.replace(/^\/?/, "")}`;
}

// Examples
joinUrl("https://mokat6.github.io/notes-docusaurus/", "clean-code/intro/");
// => https://mokat6.github.io/notes-docusaurus/clean-code/intro/

or just stick to convention and don't forget to be consistent.