PromiseOr<T, sync>

PromiseOr<T, sync extends "sync" | "async">: sync extends "async"
? T | Promise<T>
: T

A utility type for choosing between synchronous and asynchronous return values.

This is used as the return value for plugins like CustomFunction, Importer, and FileImporter so that TypeScript enforces that asynchronous plugins are only passed to compileAsync and compileStringAsync, not compile or compileString.

T
sync extends "sync" | "async"

If this is 'sync', this can only be a T. If it's 'async', this can be either a T or a Promise<T>.

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Licensed under the MIT License.
https://sass-lang.com/documentation/js-api/types/PromiseOr