A promise is a Promise Sync events without the Nasty Hacks™ David Iglesias Senior Engineer (Front-end Framework) [ twitter | speakerdeck ].com/ditman 

Event-driven code class PriceComparator { - providers = [ ProvA, ProvB, ProvC, ... ]; - handleProviderDone(provider, price) { // @todo store the price... } + computePrice (param1, param2, param3) { foreach (providers as provider) { provider.on('done', handleProviderDone); provider.computePrice(param1, param2, param3); } // @todo When all providers are done, show them } } 

● Event listeners must be subscribed before the events are fired ● Knowing when all "providers" are done is hard not straightforward Problems / hard things Naïve solutions turn into Nasty Hacks™ 

● Events with memory ○ They remember being fired ● Composed of 2 parts ○ Promise (handles event subscriptions) ○ Resolver (decides if the promise was success / fail) ● Synchronization mechanisms ○ all, any, some, chain ... Deferred to the rescue! (when.js) ...and they have been around since the '70s! 

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● Late subscription ○ Subscribe to an event seconds after it's been fired ● Synchronization ○ Synchronize 100 slowness providers Examples 

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