What are the Key Terms to Understand When Working with JavaScript Promises?

Navigating JavaScript Promise Terminology: A Comprehensive Guide

JavaScript promises play a crucial role in asynchronous programming, but their terminology can be overwhelming. Let's demystify the different terms associated with promises:

States of a Promise

According to the Promises/A specification and ES6, promises have three distinct states:

• Pending: Promise has not yet received a value, and its outcome is uncertain.
• Fulfilled: Promise has successfully acquired a result value.
• Rejected: Promise has been given a reason (typically an error) for not obtaining a result.

Settled vs. Resolved

Settled is a broader term that encompasses both fulfilled and rejected promises, meaning they have left the pending state. Resolved is often used synonymously with settled, but more precisely refers to the process of leaving the pending state.

Dynamic Transition Verbs: Fulfill, Reject

Fulfill and reject describe the actions of changing a promise from pending to fulfilled or rejected, respectively. These transitions are known as fulfillment or rejection.

Resolve: The Ambiguous Term

Resolve can be confusing. While sometimes used interchangeably with "fulfill," it is more accurately associated with settling the promise's fate (either fulfilled or rejected). The resolution of a promise means it is no longer in the pending state.

Deferring Promises

Deferring a result involves returning an asynchronous promise for the result, rather than the result itself (synchronously). It also entails returning a deferred rejection instead of throwing synchronously.

Additional Notes:

• A promise can be resolved only once.
• Resolving a promise with a "plain" value fulfills it.
• Resolving a promise with another promise "adopts" its state (i.e., resolves with its fulfillment or rejection).
• "Defer" can also refer to a method used to construct a Deferred object, which represents a promise and associated methods.

Understanding these terms is essential for effectively using JavaScript promises in asynchronous programming. By carefully employing this terminology, you can improve code readability, clarity, and efficiency.

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