Observables > Promises Everything is a stream Mark van Straten @markvanstraten 

Sync Async Single Value Promise Multiple Array Observable 

Observable Promise Lazy Eager Cancellable ✘ Chainable ✔ Testable ✔/✘ Time first class citizen ✘ Functional interface (map/filter) using Rx ✘ (does not exis 

Observables all the way Gives you control over what happens when Rx.Observable.fromEvent(myContainer, 'mousemove') .map(evt => ({ X: evt.screenX, Y: evt.screenY })) .filter(coord => coord.X % 2 === 0) .debounceTime(100) .subscribe(console.log); https://jsbin.com/tarugak/edit?js,console,output 

● Legacy code ● Interfacing with non-Observable libraries ● Ajax call to server (non cancellable) So when to use Promises? 

Why not mix ‘n match? ● Control flow using Rx + Observables ● Promises for simple actions Rx.Observable.interval(1000) .mergeMap(_ => createAccountAndReturnPassword()) .take(3) .subscribe(console.log) function createAccountAndReturnPassword(){ // todo - create account in database return Promise.resolve(Math.random() .toString(36) .substring(7) ); } 

Downside mix ‘n match ● How to test account create takes > 1 second? // promise + test + time? ● What if create account takes > 10 second? // promise + timeout ? ● What if it fails al together? // promise + retry logic? ● Cognitive overload // Is it a promise or an observable? (hint: flow/typescript) 

Recap Go play with Rx // Observables Use Observables as your interface for async functions Internally use whatever you like (promise?) Wrap promises with Rx.Observable.defer() to make them lazy