Promise

What is Promise?

A Promise is a way to represent a value that will exist (or will fail with an error) at some point in the future. This is similar to how an Optional represents a value that may or may not be there.

Using a special type to represent values that will exist in the future means that those values can be combined, transformed, and built in systematic ways. If the system knows what success and what failure look like, composing those asynchronous operations becomes much easier. For example, it becomes trivial to write reusable code that can:

• perform a chain of dependent asynchronous operations with one completion block at the end
• perform many independent asynchronous operations simultaneously with one completion block
• race many asynchronous operations and return the value of the first to complete
• retry asynchronous operations
• add a timeout to asynchronous operations

Promises are suited for any asynchronous action that can succeed or fail exactly once, such as HTTP requests. If there is an asynchronous action that can “succeed” more than once, or delivers a series of values over time instead of just one, take a look at Signals or Observables.

Overview