The easiest way to fetch JSON data when CORS is not an option but JSONP is

fetchp.js

let p = 0;
const fetchp = (url, ..._) => new Promise((resolve, reject) => {
  const src = new URL(url);
  const callback = `__json${p++}`.replace('-', '_');
  src.searchParams.set('callback', callback);
  document.head.appendChild(Object.assign(
    document.createElement('script'),
    { async: true, onerror: reject, src }
  ));
  globalThis[callback] = value => {
    delete globalThis[callback];
    resolve(value);
  };
});

Example:

fetchp('https://example.com/unique-identifier/data.json').then(
  json => {
    console.log('That is it', json);
  }
)