type TJSONPCallbackRegistryFunction<T> = (response: T) => void;

type TJSONPCallback<T> = (err: Error | null, data: T | null) => void;

declare global {
  interface Window {
    callbackRegistry: {
      [key: string]: TJSONPCallbackRegistryFunction<never>
    };
  }
}

/**
 * Simple implementation of JSONP.
 * It creates window.callbackRegistry to minimize global window pollution.
 * It uses callback to prevent adding the promise polyfill.
 */
export default function JSONP<T>(url: string, callback?: TJSONPCallback<T>, callbackName?: string): void {
  const script = document.createElement('script');
  const cbName = callbackName || 'callback';

  if (callback) {
    if (!window.callbackRegistry) {
      window.callbackRegistry = {};
    }

    const key = `cb${String(Math.random()).slice(-6)}`;

    script.src = `${url}${url.indexOf('?') === -1 ? '?' : '&'}${cbName}=callbackRegistry.${key}`;

    script.onerror = function JSONPOnError(): void {
      delete window.callbackRegistry[key];

      callback(new Error(), null);
    };

    window.callbackRegistry[key] = function JSONPCallback(response: T): void {
      delete window.callbackRegistry[key];

      callback(null, response);
    };
  } else {
    script.src = url;
  }

  document.head.removeChild(document.head.appendChild(script));
}