Asynchronous generators

We have already learned about the interface that is implemented by all iterators:

interface Iterator<T> { 
  next(value?: any): IteratorResult<T>; 
  return?(value?: any): IteratorResult<T>; 
  throw?(e?: any): IteratorResult<T>; 
} 

However, we haven't yet learned about the interface that is implemented by all asynchronous iterators:

interface AsyncIterator<T> { 
  next(value?: any): Promise<IteratorResult<T>>; 
  return?(value?: any): Promise<IteratorResult<T>>; 
  throw?(e?: any): Promise<IteratorResult<T>>; 
} 

An asynchronous iterator returns a promise every time we invoke the next method. The following code snippet demonstrates how asynchronous iterators can be very useful when used in conjunction with asynchronous functions:

let counter = 0; 
 
function doSomethingAsync() { 
    return new Promise<number>((r) => { 
        setTimeout(() => { 
            counter += 1; 
            r(counter); 
        }, 1000); 
    }); 
} 
 
async function* g1() { 
    yield await doSomethingAsync(); 
    yield await doSomethingAsync(); 
    yield await doSomethingAsync(); 
} 
 
let i = g1(); 
i.next().then((n) => console.log(n)); // 1 
i.next().then((n) => console.log(n)); // 2 
i.next().then((n) => console.log(n)); // 3