sync-promises.md

Synchronous Promise Libraries

In order to avoid missing 'data' events from flowing streams, the onFulfilled (i.e. .then function) must be called immediately, so that the next read (or another 'data' listener) can be added before the next 'data' event, which may occur immediately after the event which resolved the promise finishes emitting.

Unfortunately, this conflicts with Promises/A+ spec (although not the original Promises/A for which async was encouraged but not mandatory).

For this reason, finding Promise libraries which support synchronous operation is difficult. This document lists suitable libraries with information for choosing which one to use as a default:

broody-promises

• Can only be used synchronously inside Promise.sync function (or after calling the undocumented Promise.enter).

D.js

• Can operate synchronously if D.alwaysAsync = false (or deferred is created by defer(false)).
• Doesn't follow ES6 spec (No Promise.race, Promise.rejected instead of Promise.reject, Promise.resolved instead of Promise.resolve, promises are not instances of Promise class).

sync-promise

• Doesn't allow chaining from synchronously-resolved promises
• Throws if promise doesn't have any onRejected callbacks when rejected.
• Has a non-conformant .then method, which doesn't accept a second argument.
• Doesn't support Promise.race, Promise.resolve, or Promise.reject.

Yaku

• Operates synchronously if Yaku.nextTick is set to a synchronous function.