Differences Between Deferreds, Promises, and Futures in JavaScript

In JavaScript, deferreds, promises, and futures are commonly used to handle asynchronous operations. Each of these concepts has its own unique set of characteristics:

Deferreds

Never clearly defined in formal documentation, deferreds are typically used as the arbiter of promise resolution, implementing both resolve and reject functions. They may also be implemented as promises, enabling the use of the then function.

Promises

Promises represent the most comprehensive concept in this context. They encapsulate the result of an asynchronous operation, exposing a then function that accepts another target function. The Promises/A specification sets standards for how promises should behave, resolving ambiguity from the earlier Promises/A specification.

Futures

A term less commonly used in current discussions, futures refer to a proxy object that abstracts synchronicity and error handling without providing then functionality. FutureJS, for example, employs the term generically for such purposes.

Key Differences

• Resolution Responsibility: Deferreds are responsible for resolving promises.
• Thenability: Promises always implement the then function, while futures typically do not.
• Commonality: Deferreds are commonly used in implementations, while futures are less prevalent. Promises are the most widely used concept.

In summary, while these terms may sometimes be used interchangeably, they each hold distinct characteristics. Deferreds are the driving force behind promise resolution, promises represent the overall strategy for asynchronous operations, and futures are a less commonly used term for abstracting synchronicity.