simple jsonp that supports browser caching

fetch-jsonp.js

const fetchJSONP = ((callback, callbackName) => {
  const map = new WeakMap();
  window[callbackName] = json => {
    const script = document.currentScript;
    const fn = map.get(script);
    if (fn) {
      map.delete(script);
      fn(json);
      script.remove();
    }
  };
  return url => new Promise(resolve => {
    const script = document.createElement('script');
    url += url.indexOf('?') !== -1 ? '&' : '?';
    url += callback + '=' + callbackName;
    map.set(script, resolve);
    script.src = url;
    document.body.append(script);
  });
})('callback', '_jsonp_global_');

jsonp-polyfill.js

fetch = ((nativeFetch) => {
  return (url, options) => {
    if (options && options.method.toUpperCase() === 'JSONP') {
      return fetchJSONP(url)
        .then(json => new Response(JSON.stringify(json)));
    }
    return nativeFetch(url, options);
  };
})(fetch);

test.js

// test on jsfiddle
// https://jsfiddle.net/zakariamouhid/pyzagmx7/2/

// it depends on document.currentscript
// https://caniuse.com/document-currentscript

// Examples

const url = 'https://zakariamouhid.blogspot.com/feeds/posts/default?alt=json-in-script&max-results=0';

fetchJSONP(url).then(json => {
  console.log(json.version);
});

fetch(url, { method: 'JSONP' })
  .then(res => res.json())
  .then(json => {
  console.log(json.version);
});