Dart: How to return Future<void>

You don't need to return anything manually, since an async function will only return when the function is actually done.

Looking at your examples you are missing the async keyword, which means you need to write the following instead:

Future<void> deleteAll(List stuff) async {
    stuff.forEach( s => delete(s));
}

Future<void> delete(Stuff s) async {
   ....
   await file.writeAsString(jsonEncode(...));
}

There is no need to return anything, since void is nothing, as the name implies.

Also make sure you call deleteAll and writeAsString() using await.

You can't do that with forEach.

But you can use Future.wait and .map like this

Future<void> deleteAll(List stuff) {
  return Future.wait(stuff.map((s) => delete(s)));
}

Future<void> delete(Stuff s) async{
  ....
  await file.writeAsString(jsonEncode(...));
}

When to use async keyword:

You can use async when your function uses await keyword inside.

So when to use await keyword:

1. when you want to get the result from an asynchronous function and want do some logic on the result

Future<int> fetchCountAndValidate() asycn{
  final result = await fetchCountFromServer();
  if(result == null)
    return 0;
  else
   return result;
}

2. When you want to call multiple asynchronous function

Future<int> fetchTotalCount() asycn{
  final result1 = await fetchCount1FromServer();
  final result2 = await fetchCount2FromServer();
  return result1 + result2;
}

When you don't need async or await:

1. When you just calling another asynchronous function

Future<int> getCount(){
  //some synchronous logic
  final requestBody = {
    "countFor": "..."
  };
  return fetchCountFromServer(requestBody); //this is an asynchronous function which returns `Future<int>`
}

2. For some rare cases we doesn't care about the completion of asynchronous function

void sendLogoutSignal(){
  http.post(url, {"username" : "id0001"});
}