async-iterator-cancellation.js

const delay = ms => new Promise(resolve => setTimeout(resolve, ms));

async function* delayedRange() {
  try {
    for(let i = 1; i <= 1000; i++) {
      await delay(100);
      yield i;
    }
  } catch (e) {
    console.log("Oh no error in iterator", e);
  } finally {
    console.log("OMG, finally blocks run on termination"); 
  }
}

const range = delayedRange();

console.log(await range.next()); // { value: 1, done: false }
console.log(await range.next()); // { value 2, done: false }
// range.return(); // this runs the `finally` block, logs and finishes the generator.
range.throw(new RangeError('fun')); // this runs *both* the catch and the finally

// In a for await loop, `break`ing the loop calls `.return` implicitly so loops support cancellation
// In a for await loop, `throw`ing similarly propagates the error - funnily it's just this way because 
// cancellation progress stopped "mid way" and because of the duality proposal.