People do not use the terms consistently.

The difference is SUPPOSED to be that a Future is read-only while a Promise is settable. And should be set at most once! So you can use a Promise as a Future, but you can't always use a Future as a Promise.

Linguistically you can understand this as, "I Promise you'll see a Future answer." I need to be able to write to the Promise to fulfill it. You can only hope that some day the Future will arrive.

In Javascript: Promise is read-only (public interface), Deferred is writable (private)

on JVM: Future is read-only (public), Promise is writable (private)

The writable interface has "resolve" and "reject" to return a value. The readable interface has "then", "error", "finally" etc which let you compose and combine functions that deal in asynchronous values.

The best Javascript implementation of promises is Bluebird, which doesn't expose a Deferred instance to avoid confusion, it is very nice. bluebirdjs.com/docs/api-reference.html

As far as I can tell, there's two differences. 1) a Promise typically has a "public" API and a "private" API -- the private API has read/write access and the public API has only read access. This way you can return the public API object to consumers of a method/library without worrying about them mutating the promise. 2) Promises tend to be more...composable? Java's "CompletableFuture" API fails on the #1 point I made above, but otherwise has a whole slew of then* methods that aren't on Java's Future. Then again, that might just be because Java's Future is mind-numbingly simplistic.