Transformation Operators

Transformation Operators

• Transform values emitted from the source observable
• Some work on higher order observables
• The map operator is a transformation operator!

The scan Operator

• Similar to Array#reduce()
• Evaluate each item emitted from the source observable while maintaining an accumulated value

numbers$.pipe(scan((total, next) => total + next, 0)).subscribe();

Higher-Order Observables

clicks$.pipe(map(() => timer(1_000)));

• Note that the type of is now Observable<Observable<number>>
• This is called a higher-order observable
• It's an observable of observables!

clicks$.pipe(
  map(() => timer(1_000)),
  mergeAll()
);

• The mergeAll operator "flattens" the observable
• It subscribes to each observable emitted by the map function

The mergeMap Operator

• Short hand for map+mergeAll
• Map items from the source observable to new observables, then flatten the result

click$.pipe(mergeMap(() => ajax('/api/v1/books.json')));