Promise ALL the things!

Code examples for Vilnius JS meetup, November 20, 2012

Problem

Async feels hard. It is only natural to want things to look-and-feel sync(ish).

When building UI, on rarely goes into deeply nested callbacks - interactions usually result in one "response".

When building in node, programs need to gather data from multiple data sources and combine it.

Simple callbacks are OK until...

• You need proper error handling
• You need to load data in a sequence
• You need to load data in parallel

Async.js style libs are OK until...

• You need to share loaded data

Events are different

• Each event can happen multiple times with multiple values
  • A promise gets fulfilled (resolved or rejected) exactly once
• Event's are not "replayed" to subscribers
  • Once a promise is fulfilled - every subscriber will get exactly the same value/reason.

Oh yeah... so what's a promise anyways?

In computer science, future, promise, and delay refer to bla bla synchronizing bla bla object bla bla proxy bla bla.

• wat http://en.wikipedia.org/wiki/Futures_and_promises

[In JavaScript science] A promise represents the eventual value returned from the single completion of an operation.

• wat http://wiki.commonjs.org/wiki/Promises/A

[In non-scientific daily life] A promise has a .then(), which takes subscribers and calls them when it is resolved/rejected.

• .then()s can be chained and pass on then values/promises from their subscribers.

Gotchas and features

• always ALWAYS call done() // FKA end()
• execution order guarantees
  • callback is always async
  • chained callbacks always in order
  • see: https://github.com/promises-aplus/promises-spec
• execution order gotchas
  • when you have multiple subscribers, they are executed breadth-first
  • fail/fin/then/spread/etc in Q are all really just a fancy then()
• not all promises are created equal
  • value and state should not change once fulfilled
  • chain-ability is key
  • see: https://gist.github.com/3889970

Async is fun

Love it.