Add Promisable type and allow it as an argument to useAsync.

src/Async.tsx

@@ -1,5 +1,10 @@
 import React, {useEffect, useState} from "react";
 
+/**
+ * A type that can be returned by a promise or as is.
+ */
+export type Promisable<T> = T|Promise<T>;
+
 /**
  * Asynchronous data state.
  */
@@ -12,7 +17,7 @@ interface AsyncState<T>
 	/**
 	 * The promise which is retrieved (or has retrieved) data.
 	 */
-	promise: Promise<T>;
+	promise: Promisable<T>;
 	/**
 	 * Error thrown by the promise.
 	 */
@@ -33,20 +38,20 @@ export type PromiseFn<T> = () => Promise<T>;
  * @param promise The promise or a function that produces a promise.
  * @param deps When one of the `deps` change, it will wait for the promise again.
  */
-export function useAsync<T>(promise: Promise<T>|PromiseFn<T>, deps: any[] = []): AsyncState<T>
+export function useAsync<T>(promise: Promisable<T>|PromiseFn<T>, deps: any[] = []): AsyncState<T>
 {
 	// Get the actual promise from the function if there is one.
 	if ((promise as PromiseFn<T>)?.call)
 		promise = (promise as PromiseFn<T>)();
-	else
+	else if (promise instanceof Promise)
 		promise = Promise.race([promise as Promise<T>]);
 
 	// The async state.
 	const [state, setState] = useState<AsyncState<T>>({
-		pending: true,
-		promise: promise,
+		pending: promise instanceof Promise,
+		promise: promise as Promisable<T>,
 		error: undefined,
-		data: undefined,
+		data: promise instanceof Promise ? undefined : promise as T,
 	});
 
 	/**
@@ -60,7 +65,11 @@ export function useAsync<T>(promise: Promise<T>|PromiseFn<T>, deps: any[] = []):
 
 	// Reconfigure the promise when any deps have changed.
 	useEffect(() => {
-		(promise as Promise<T>).then((result) => {
+		if (!(promise instanceof Promise))
+			// If it's not a promise, there is nothing to wait for.
+			return;
+
+		promise.then((result) => {
 			// When there is a result, disable pending state and set retrieved data, without error.
 			updateState({
 				pending: false,
@@ -79,7 +88,7 @@ export function useAsync<T>(promise: Promise<T>|PromiseFn<T>, deps: any[] = []):
 		// Promise is ready: reset the state to pending with the configured promise, without data and error.
 		updateState({
 			pending: true,
-			promise: promise as Promise<T>,
+			promise: promise,
 			error: undefined,
 			data: undefined,
 		});