cancel-with-null-fulfillment.js

//canceled promise resolving with no response:
let pendingPromise = null
function fetchOnce(url) {
  if (pendingPromise) {
    pendingPromise.cancel()
  }
  return http.get('/foo').then(res => {
    pendingPromise = null
    return res
  }).catch(e => {
    pendingPromise = null
    throw e
  })
}

function loadData() {
  return (dispatch) => 
    fetchOnce().then(res => {
      if (res) dispatch({
        type: 'AJAX_DONE',
        payload: { res }
      })
    }).catch(e => {
      dispatch({ type: 'AJAX_ERROR', error: true, payload: e })
    })
  }
}

cancel-with-rejection.js

//canceled promise rejecting with CancellationError
let pendingPromise = null
function fetchOnce(url) {
  if (pendingPromise) {
    pendingPromise.cancel()
  }
  return http.get('/foo').then(res => {
    pendingPromise = null
    return res
  }).catch(e => {
    pendingPromise = null
    throw e
  })
}

function loadData() {
  return (dispatch) => 
    fetchOnce().then(res => {
      //res wont be null
      dispatch({
        type: 'AJAX_DONE',
        payload: { res }
      })
    }).catch(e => {
      dispatch({ type: 'AJAX_ERROR', error: true, payload: e })
    }).catch(Promise.CancellationError, () => {
      //do nothing, promise was canceled
    })
  }
}