java async task

.map

jshell> var t1 = CompletableFuture.completedFuture(100)
t1 ==> java.util.concurrent.CompletableFuture@25f38edc[Completed normally]

jshell> var t2 = t1.thenApply(x -> x * 2)
t2 ==> java.util.concurrent.CompletableFuture@1a86f2f1[Completed normally]

jshell> t2.get()
$14 ==> 200

.fmap

jshell> var t1 = CompletableFuture.completedFuture(100)
t1 ==> java.util.concurrent.CompletableFuture@506c589e[Completed normally]

jshell> var t2 = CompletableFuture.completedFuture(200)
t2 ==> java.util.concurrent.CompletableFuture@69d0a921[Completed normally]

jshell> var result = t1.thenCompose($ -> t2)
result ==> java.util.concurrent.CompletableFuture@7aec35a[Completed normally]

jshell> result.get()
$19 ==> 200

javascript equivalent

> var t1 = Promise.resolve(100)
undefined
> var t2 = t1.then(x => x * 2)
undefined
> t2
Promise {
  200,
  domain: 
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }