Cancel token discussion points

dot-promise-is-for-adapting-old-apis.js

function xhrAdapted(url, { cancelToken } = {}) {
  const xhr = new XMLHttpRequest();
  return new Promise((resolve, reject, cancel) => {
    xhr.addEventListener("load", () => resolve(xhr.responseText));
    xhr.addEventListener("error", () => reject(new Error("could not XHR")));
    
    if (!cancelToken) return;
    cancelToken.promise.then(reason => {
      cancel(reason);
      xhr.abort();
    });
  });
}

you-have-to-keep-passing-it.js

// With "tasks"

startSpinner();

const p = fetch(url)
  .then(r => r.json())
  .then(data => fetch(data.otherURL))
  .then(r => r.text())
  .then(text => updateUI(text))
  .catch(err => showUIError())
  .finally(stopSpinner);

cancelButton.onclick = () => p.cancel();

// With cancel tokens:

startSpinner();

let cancel;
const cancelToken = new CancelToken(c => cancel = c);

fetch("url", { cancelToken })
  .then(r => r.json({ cancelToken }))
  .then(data => fetch(data.otherURL, { cancelToken }))
  .then(r => r.text({ cancelToken }))
  .then(text => updateUI(text))
  .catch(err => showUIError())
  .finally(stopSpinner);

cancelButton.onclick = cancel;