How can I find out if a javascript iterator terminates early?

Lets say I have a generator:

function* source() {
  yield "hello"; yield "world";
}

I create the iterable, iterate with a for-loop, and then break out of the loop before the iterator fully completes (returns done).

function run() {
  for (let item of source()) {
    console.log(item);
    break;
  }
}

Question: How can I find out, from the iterable-side, that the iterator terminated early?

There doesn't seem to be any feedback if you try to do this directly in the generator itself:

function* source2() {
  try {
    let result = yield "hello";
    console.log("foo");
  } catch (err) {
    console.log("bar");
  }
}

... neither "foo" nor "bar" is logged.

There's a much simpler way to do this: use a finally block.

function *source() {
  let i;

  try {
    for(i = 0; i < 5; i++)
      yield i;
  }
  finally {
    if(i !== 5)
      console.log('  terminated early');
  }
}

console.log('First:')

for(const val of source()) {
  console.log(`  ${val}`);
}

console.log('Second:')

for(const val of source()) {
  console.log(`  ${val}`);

  if(val > 2)
    break;
}

...yields:

First:
  0
  1
  2
  3
  4
Second:
  0
  1
  2
  3
  terminated early