Question 1

What are the three states of a Promise in JavaScript?

Accepted Answer

Promises have three mutually exclusive states: pending, fulfilled, and rejected, as defined in the promises-unwrapping spec. A promise is pending if unsettled, fulfilled with a value, or rejected with a reason, governing how it reacts to then calls.

Question 2

How does Promise.all work internally?

Accepted Answer

Promise.all creates a new promise that fulfills with an array of results when all input promises fulfill, or rejects on the first rejection. Internally, it uses PromiseReaction records and a counter to track elements, as specified in the Promise.all Resolve Element Functions section.

Question 3

What's the difference between Promise.all and Promise.race?

Accepted Answer

Promise.all waits for all promises to settle and returns an array of results, while Promise.race returns the result of the first promise to settle. The spec defines distinct algorithms for both, with Promise.race settling immediately on the first resolution or rejection.

Question 4

How to create a custom Promise subclass in JavaScript?

Accepted Answer

To subclass Promise, extend the Promise constructor and ensure proper inheritance using the NewPromiseCapability operation. The spec supports this by allowing any constructor that follows Promise parameter conventions, as detailed in the Subclassing Support section.

Question 5

What are PromiseCapability records used for?

Accepted Answer

PromiseCapability records encapsulate a promise object with its resolve and reject functions, used internally by abstract operations like NewPromiseCapability. They enable consistent handling of promise resolution and rejection across the specification's machinery.

Question 6

Is the promises-unwrapping spec still relevant today?

Accepted Answer

The promises-unwrapping spec was the basis for the ECMAScript 6 Promise standard and is historically significant. However, the README advises referencing the official draft specification for current implementations, as this repo is a staging ground for fixes.

Question 7

How to handle errors in promise chains?

Accepted Answer

Errors propagate through promise chains via rejection handlers, specified using PromiseReaction records. The Thrower handler automatically throws errors, and the catch method is defined as a shorthand for then with an undefined onFulfilled, as per the Promise.prototype.catch section.

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