Promises
vs
Observables
- Callbacks
- Generators
- Async / await
- Promises
- Observables
getLoggedInUser((err, user) => {
if (err) {
displayError();
return;
}
getMoviesForUser(user, (err, userMovies) => {
if (err) {
displayError();
return;
}
getMovieDetails(userMovies[0], (err, movieDetails) => {
if (err) {
displayError();
return;
}
displayMovie(movieDetails);
});
});
});Callbacks
Promises
Value
function createUser(user) {
return new Promise((resolve, reject) => {
doCreateUser(user, (err, result) => {
if(err) {
reject(err);
} else {
resolve(result);
}
});
});
}createUser(user)
.then(result => display(result))
.catch(err => displayError());function doSomething(user) {
return new Promise((resolve, reject) => {
console.log(`I'm doing some stuff...`);
});
}
doSomething();function doSomething() {
return new Promise((resolve, reject) => {
resolve(1);
resolve(2);
reject(3);
});
}
doSomething().then(onSuccess, onFailure);- Are part of EcmaScript specification
- Can only be resolved once
- Are not guaranteed to be resolved
- Are not lazy
- Cannot be cancelled
Promises
Observables
RxJS
RxJS
import { Observable } from 'rxjs';
const hello = Observable.create(function(observer) {
observer.next('Hello');
observer.next('World');
observer.complete();
});
hello.subscribe(val => console.log(val));const observable = getRealTimeData();Hot'n'Cold
const source = new Observable((observer) => {
const socket = new WebSocket('ws://my-url');
socket.addEventListener('message', (event) => {
observer.next(event);
});
return () => socket.close();
});(not the Katy Perry song...)
const socket = new WebSocket('ws://my-url');
const source = new Observable((observer) => {
socket.addEventListener('message', (event) => {
observer.next(event);
});
});import { Observable } from 'rxjs';
const subscription = getData().subscribe(data => console.log(data));
subscription.unsubscribe();- Are not part of EcmaScript specification
- Can have multiple values
- Are not guaranteed to be resolved
- Can be lazy (hot vs cold)
- Can be cancelled
Observables
Promises
vs
Observables
- You have only one event
- Work must be done only once
- Your data could change over time
- You will receive several events
- You have complex operations
- You may need to cancel the operation
Thank you!
Promises vs Observables
By ereold
