Async/Await allow asynchronous code to be written as straight-line code.
If you write code that makes a series of such asynchronous calls in a callback-based API, you will easily fall into callback hell.
Refactoring the code using async/await, it can be written
in a clear and straightforward manner as follows.
async/await functions in parallel
The following code executes each
fetchTodoDetail(id:) function sequentially.
To call each fetchTodoDetail(id:) function in parallel,
use async let to defer the calls, then use
await to resolve the values in later lines. They will be
executed in parallel at the line with the await.