Today I Learned

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Create A Cancelable Promise With PCancelable

If an async task takes a really long time and we find out in the middle of its execution that we no longer want the results of that activity, it would be nice to be able to cancel that promise. There was a proposal for cancelable promises, but it has since been withdrawn. There is an alternative though.

The p-cancelable package provides a promise wrapper that acts as a cancelable promise.

import PCancelable from 'p-cancelable';

const fetchPromise = new PCancelable((resolve, reject, onCancel) => {
  setTimeout(() => {
    resolve({ ok: true, data: [1, 2, 3] });
  }, 10000);

  onCancel(() => {
    console.log('Promise is being canceled');
  });
});

fetchPromise.then(response => {
  console.log('Promise Resolved: ', response.data);
}).catch(err => {
  console.log('Promise Rejected: ', err);
});

fetchPromise.cancel();

We can create a promise in a familiar manner. We get an additional argument — the onCancel function. This is a function that will be executed if cancel is called before the promises has resolved or rejected. In the above example, the fetchPromise.cancel() line will be invoked before the setTimeout resolves. The promise will be canceled, causing a rejection which will push us into the catch handler.