I guess this is not all that common, but e.g. I have this helper where I want a function to be sync when passed a sync callback and async when passed an async callback

function split(genFn) {
  function runSync(...args) {
    const it = genFn(...args);
    let next = it.next();
    while (true) {
      if (next.done) return next.value;
      next = it.next(next.value);
    }
  }

  async function runAsync(...args) {
    const it = genFn(...args);
    let next = it.next();
    while (true) {
      if (next.done) {
        return await next.value;
      }
      try {
        next = it.next(await next.value);
      } catch (err) {
        next = it.throw(err);
      }
    }
  }

  return { sync: runSync, async: runAsync };
}

I am imagining something like

let treeId = new AsyncContext.Variable();

async function fetchChildren(node) {
  console.log(`fetching children for node in tree from file ${treeId.get()}`);
  return thing;
}

async function* walk(node) {
  for (let child of await fetchChildren(node)) {
    yield child;
    if (child.hasChildren) {
      yield* walk(child);
    }
  }
}

let runner = new TaskRunner({ workers: 4 });

document.querySelector('#process').addEventListener('click', async () => {
  let file = document.querySelector('#file').files[0];
  let tree = parse(await file.bytes());
  treeId.run(file.name, () => {
    runner.add(walk(tree).map(console.log));
  });
});