Rejections

Installation

npm install --save rejections

Motivation

I do love Promises and use them everywhere. I promise. =) It is recommended practice to use instances of Error as a rejection reason of Promises. Some libs (e.g. bluebird) will even show you a warning in case you reject Promise with non-error. Straightforward solution is to use subclasses of Errors. But it leads to some complications.

Complication 1

In case you want serialize/deserialize rejection to pass it from server to client, or to save it in Redux store, you'll have to write custom serialization/deserialization code, which I feel lazy to do every time.

Complication 2

If you want to take action based on rejection class (which is the whole point behind using Error subclasses as rejections), and you are using Babel to make your code ES5-browsers compatible, then you'll have to deal with wierd issue:

class UserNotFound extends Error {
  contstructor(username) {
    super();
    this.username = username;
  }
}

new UserNotFound('Fedor') instanceof UserNotFound // will be false

To deal with both compilcatins you'll have to introduce some kind of type filed in your Error subclasses.

Solution

So why use subclasses at all? With some inspiration from redux-actions this lib provides two main functions createRejection, handleRejections and one utility function parseRejection.

createRejection(type)

Returns rejection creator for given rejection type. Creator will return Error subclass with provided payload.

const userNotFound = createRejection('user-not-found');
const networkProblem = createRejection('network-problem');

const r1 = userNotFound('Fedor');
const r2 = networkProblem({ host: 'google.com', status: 404 });

r1 instanceof Error; // true
r2 instanceof Error; // true

JSON.stringify(r1); // {"type":"user-not-found","payload":"Fedor"}
JSON.stringify(r2); // {"type":"network-problem","payload":{"host":"google.com","status":404}}

handleRejections(rejectionsMap, defaultHandler = IdentityRejector)

Creates rejections handler for given rejection types. In case no matching handler found in rejectionsMap provided rejection will be passed to defaultHandler. In case defaultHandler is not provided function re-rejecting input will be used (i.e. r => Promise.reject(r)). The result of matching handler will be returned for convinience.

const knownIssue = createRejection('known-issue');
const otherKnownIssue = createRejection('other-known-issue');

...

API.getCriticalData()
    .then(
        parseIt, // make data usable for your app
        handleRejections({
          [knownIssue]: rej => tryToRecover(rej),
          [otherKnownIssue]: rej => tryToRecoverFromAnotherIssue(rej)
        }) // unknown problem, pass reject further
    )
    .then(useIt)
    .catch(rej => {
      // was not able to recover
    });
    
...

API.getNotCriticalData()
    .then(
        parseIt, // make data usable for your app
        handleRejections({
          [knownIssue]: rej => tryToRecover(rej),
          [otherKnownIssue]: rej => tryToRecoverFromAnotherIssue(rej)
        }, rej => replaceWithDummyData()) 
    )
    .then(useIt);
    

parseRejection(raw)

Creates rejection instance from the given raw rejection data, which should be of following shape:

{
  type: String,
  payload: Anything | undefined,
}

In case raw rejection does not match that shape, rejection with type 'unknown' will be created. raw argument will be passed as its payload:

const r1 = parseRejection({ type: 'known', payload: { field: 'value' } });
// r1.type is 'known'
// r1.payload is { field: 'value' }

const r2 = parseRejection('We have failed badly');
// r2.type is 'unknown'
// r2.payload is 'We have failed badly'

const r3 = parseRejection({ type: 'not-so-well-formatted', data: 'I will dissapear!' });
// r3.type is 'not-so-well-formatted'
// r3.payload is undefined

License

Apache-2.0