Cancellable fetch

foo.ts

type CancellableFetchResult = { aborted: true } | { aborted: false; responseText: string };
export type CancellableFetchReturn = {
  completePromise: Promise<CancellableFetchResult>;
  abort: Function;
};

// Returns a promise API and the original XHR for abortion.
export const cancellableFetch = (
  url: string,
  options: RequestInit = {},
): CancellableFetchReturn => {
  const xhr = new XMLHttpRequest();
  const completePromise = new Promise<CancellableFetchResult>((resolve, reject) => {
    xhr.addEventListener('error', () => {
      reject(new Error('XHR error'));
    });
    xhr.addEventListener('abort', () => {
      resolve({ aborted: true });
    });
    xhr.addEventListener('load', () => {
      resolve({ aborted: false, responseText: xhr.responseText });
    });
    const method = options.method !== undefined ? options.method : 'GET';
    xhr.open(method, url, true);
    xhr.send();
  });

  const abort = () => xhr.abort();

  return { completePromise, abort };
};