What are the Differences Between Deferreds, Promises, and Futures in JavaScript?

Differences Between Deferreds, Promises, and Futures in JavaScript

Introduction

In JavaScript, asynchronous programming can be handled through various concepts such as deferreds, promises, and futures. This article aims to clarify the nuances and differences among these terms, providing a comprehensive understanding of their roles in async programming.

Terminology

• Deferred:

  • Not explicitly defined in popular references, but commonly used in implementations as the orchestrator of promise resolution.
  • May or may not have promise-like capabilities.

• Promise:

  • The overarching term encompassing the asynchronous programming strategy.
  • Stores the result of an asynchronous function and provides a then function for chaining subsequent operations.

• Future:

  • A term that is gradually being phased out in favor of "promise."
  • Was used in some implementations to represent abstract asynchronous operations, but lacks the chaining capabilities provided by promises.

Comparison:

Feature	Deferred	Promise	Future
Resolution Capability	Yes (via methods)	No	No
Promise-Like Functionality	May or may not have	Yes	No
Presence in Standard	Not explicitly mentioned	Yes	Deprecated
Usage in Implementations	Common as resolution orchestrator	Always present, with resolution handled externally	Uncommon, being replaced by "promise"
Popularity	Widely used	Common terminology	Declining usage

Conclusion:

While the terms deferred, promise, and future may be somewhat interchangeable in the context of asynchronous programming, their specific semantics and usage patterns vary across implementations and usage guidelines. It's essential to understand these differences to effectively leverage the capabilities of these concepts in JavaScript.

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