Synchronous callback implementation
An Auth.js callback can compute its result without I/O.
Auth.js · Type Alias
Awaitable
Awaitable<T> is Auth.js's public type for a result that can be supplied immediately as T or asynchronously through PromiseLike<T>. It lets callbacks and provider hooks support either synchronous or asynchronous implementations.
How it works
Awaitable<T> is a union that permits either a value of type T or a PromiseLike<T>. Auth.js uses it for extension points such as callbacks, allowing implementations to complete immediately or return a thenable that resolves to the declared result.
Syntax
type Awaitable<T> = T | PromiseLike<T>;Key points
- Awaitable is documented as a public type in the core package's types module.
- Its generic T describes the result type shared by both allowed branches.
- Auth.js applies Awaitable to callback return types such as redirect, whose result is a string.
- JWT callbacks can return either immediately or asynchronously, with null or JWT as the result.
Common mistakes
- Awaitable is broader than Promise<T>: synchronous values are valid, and the asynchronous branch is PromiseLike<T>, not only native Promise<T>.
Example: Accepting synchronous or asynchronous values
type Awaitable<T> = T | PromiseLike<T>;
const immediate: Awaitable<string> = "done";
const deferred: Awaitable<string> = Promise.resolve("done");
async function readValue(value: Awaitable<string>): Promise<string> {
return await value;
}
console.log(await readValue(immediate));
console.log(await readValue(deferred));The same function accepts either an immediate string or a promise-like string, then normalizes both with await.
When to use it
Asynchronous extension point
A callback or provider hook needs database, network, or other asynchronous work.
Flexible API typing
A library function should accept either a direct result or a thenable.
Interview questions
QWhat does Awaitable<T> represent?
Awaitable<T> means T or PromiseLike<T>. It lets an Auth.js callback return its result directly or asynchronously while preserving the result type.
Follow-up: How would a consumer handle both forms? Use await: JavaScript assimilates thenables, so awaiting the value produces T.
QWhy would Auth.js use Awaitable for callbacks?
A callback typed as Awaitable<string> may return a string immediately or a promise-like object resolving to a string. This supports both synchronous and asynchronous implementations without separate callback types.
Follow-up: Why is this useful in library APIs? Callers can provide lightweight synchronous handlers while still supporting I/O-bound handlers.
QWhat is the exact declaration of Awaitable?
The declaration is type Awaitable<T> = T | PromiseLike<T>. The generic parameter T is the eventual logical result, while the union permits either immediate or deferred delivery.
Follow-up: How does it differ from Promise<T>? Promise<T> requires a promise object; Awaitable<T> also accepts T directly and any compatible thenable.
Related
AwaitedPromiseLikeAuth.js callbacksProfileCallback