JavaScript Promises: Navigating the Terminology Maze
Promises in JavaScript have given rise to a tapestry of terms that can become overwhelming. To unravel this terminology, let's dive into the heart of Promises/A specification and ES6.
The Three States:
Promises exist in one of three states:
- Pending: The promise hasn't reached a concrete value yet, leaving its outcome uncertain.
- Fulfilled: The promise has successfully acquired a result value.
- Rejected: The promise has received a reason (usually an error) explaining why no result could be obtained.
Settled, Fulfilled, and Resolved
The term settled encompasses both fulfilled and rejected states, indicating that the promise is no longer in a pending state. Fulfilled specifically denotes the successful acquisition of a result, while rejected signifies the failure to acquire a result.
Resolve
Resolve is a contentious term that can be interpreted in two ways:
- Synonymous with "Fulfill": In some contexts, resolve is used interchangeably with "fulfill."
- Settling the Fate: A more accurate understanding of resolve is as settling the promise's fate, either toward fulfillment or rejection.
Promises Resolved with Promises
Resolving a promise with a promise (or thenable) means adopting the state of that promise. This leads to the possibility of resolving a promise's fate without knowing whether it will be fulfilled or rejected.
Defer
Defer refers to returning an asynchronous promise for a result instead of the result itself (synchronously). This ensures that rejections are handled through deferred rejection rather than synchronous exceptions.
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