The idea of the Asynchronous State Handler is to combine asynchronous operations with an explicit loading and error state. While an asynchronous operation is running, the status is loading. It ends either in an error state or with the actual result of the operation. Thinking of a generic operation result T, the Asynchronous State Handler is a function that returns T | 'loading' | 'error'. Using a union type underlines that the result is always exactly one state. Using string literal types for loading and error state is simple but descriptive. They can be replaced by other types if required.

Composite: Basic Idea
The first thing we need to look at is the definition offered by the book from the Gang of Four:

“Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.”