A promise generator is analogous in all ways to a plain generator. Promise generators implement yield, return, and throw. The return and throw methods both terminate the stream. Yield accepts a value. They all return promises for an acknowledgement iteration from the consumer. Waiting for this promise to settle causes the producer to idle long enough for the consumer to process the data.

One can increase the number of promises that can be held in flight by a promise buffer. The buffer constructor takes a length argument that primes the acknowledgement queue, allowing you to send that number of values before having to wait for the consumer to flush.

var buffer = new Buffer(1024);
function fibStream(a, b) {
    return buffer.in.yield(a)
    .then(function () {
        return fibStream(b, a + b);
    });
}
fibStream(1, 1).done();
return buffer.out;

If the consumer would like to terminate the producer prematurely, it calls the throw method on the corresponding promise iterator. This will eventually propagate back to the promise returned by the generator’s yield, return, or throw.

buffer.out.throw(new Error("That's enough, thanks"));