Coming from Other Languages

This page describes parallels of using promises in other languages. Promises as a pattern are very common in other languages and knowing what they map to in other languages might help you with grasping them conceptually


A promise is similar to a C# Task. They both represent the result of an operation.

A promise's then method is similar to a Task's ContinueWith method in that both allow attaching a continuation to the promise. Bluebird's Promise.coroutine is analogous to C#'s async/await syntax.

A TaskCompletionSource is analogous to the promise constructor. Although usually promisification is preferred (see the API reference or working with callbacks section).

Task.FromResult is analogous to Promise.resolve.

The difference between a Task and a promise are that a task might not be started and might require a .Start call where a promise always represents an already started operation.

In addition promises are always unwrapped. A promise implicitly has Task.Unwrap called on it - that is, promises perform recursive assimilation of promises within them.

See this question on StackOverflow for more differences.


A bluebird promise is similar to a Scala Future. A scala Promise is similar to how the promise constructor can be used (previously, to a bluebird Deferred).

Just like a future, a promise represents a value over time. The value can resolve to either a fulfilled (ok completion) or rejected (error completion) state.

Where blocking on a Future in scala is discouraged, in JavaScript it's downright impossible.

In addition promises are always unwrapped. That is, promises perform recursive assimilation of promises within them. You can't have a Promise<Promise<T>> where a Future[Future[T]] is valid in Scala.

See this question on StackOverflow for more differences.


A promise is similar to a Twisted Deferred object. In fact the first JavaScript implementations of promises were based on it. However, the APIs have diverged since. The mental model is still very similar.

A promise is not similar to a Python concurrent.Future which does not chain actions.

Asyncio coroutines are similar to bluebird coroutines in what they let you do, however bluebird coroutines also enable functional-style chaining.


A bluebird promise is similar to a std::future and the promise constructor is similar to an std::promise although it should rarely be used in practice (see the promisification section).

However, a bluebird promise is more powerful than the current implementation of std::future since while chaining has been discussed it is not yet implemented. Promises can be chained together.

Boost futures expose a .then method similar to promises and allow this functionality.


A promise is a monadic construct with .then filling the role of >>= (bind). The major difference is that .then performs recursive assimilation which acts like a flatMap or a map. The type signature of then is quote complicated. If we omit the error argument and not throw - it's similar to:

then::Promise a -> (a -> (Either (Promise b) b)) -> Promise b

That is, you can return either a promise or a plain value from a then without wrapping it.

Promises perform a role similar to IO in that they allow for easy chaining of asynchronous non-blocking operations. Promise.coroutine can be seen as similar to do notation although in practice it's not an accurate comparison.


A promise is similar to a guava Future with chain being similar to then.

If your'e familiar with Java 8 lambdas, you can think of a promise as a Future you can map to another future.

Android Java

Several popular Android libraries use promises - for example the Parse Java API returns Tasks which are similar to JavaScript promises.


If you're familiar with PromiseKit, it is based on a same specification bluebird is based on so the API should feel familiar right away.

© 2013–2017 Petka Antonov
Licensed under the MIT License.