Promise vs Observable

One-minute read

1:1

  • a Promise is eager, whereas an Observable is lazy,
  • a Promise is always asynchronous, while an Observable can be either synchronous or asynchronous,
  • a Promise can provide a single value, whereas an Observable is a stream of values (from 0 to multiple values),
  • you can apply RxJS operators to an Observable to get a new tailored stream (categorized as: creation[from, range], transformation[map, scan ], filtering[first, last], joining[mergeall, startwith], multicasting[publish, share], error handling[catchError, retry], utility[delay, timeout])

Cheat sheet

The following code snippets illustrate how the same kind of operation is defined using observables and promises.

Operation Observable Promise
Creation new Observable((observer) => { observer.next(123);}); new Promise((resolve, reject) => { resolve(123);});
Transform obs.pipe(map((value) => value * 2)); promise.then((value) => value * 2);
Subscribe sub = obs.subscribe((value) => { console.log(value)}); promise.then((value) => { console.log(value); });
Unsubscribe sub.unsubscribe(); Implied by promise resolution.