What is Future data type in Dart?

There are two ways of getting a value from a Future .

Future<T> means that the result of the asynchronous operation will be of type T . For example, if a function returns Future<String> , this means that in the future, it will provide a string to you. For the sake of practicing, we can use one of the Future constructors to create our own Future object.

If a future doesn’t produce a usable value , then the future’s type is Future<void> . A future can be in one of two states: uncompleted or completed. When you call a function that returns a future, the function queues up work to be done and returns an uncompleted future.

Essentially, by making some function f() return a Future<Null> , you are making await f() a synonym of null in your program ! That’s dangerous because you can use null as if it were absolutely anything, absolutely any type.30 Eki 2018

FutureOr<T> class Null safety A type representing values that are either Future<T> or T . This class declaration is a public stand-in for an internal future-or-value generic type, which is not a class type.