Add PromiseValue type (#75)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -15,6 +15,7 @@ export {Promisable} from './source/promisable';
 export {Opaque} from './source/opaque';
 export {SetOptional} from './source/set-optional';
 export {SetRequired} from './source/set-required';
+export {PromiseValue} from './source/promise-value';
 
 // Miscellaneous
 export {PackageJson} from './source/package-json';

readme.md

@@ -74,6 +74,7 @@ Click the type names for complete docs.
 - [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/).
 - [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional.
 - [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required.
+- [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise` type.
 
 ### Miscellaneous
 

source/promise-value.d.ts

@@ -0,0 +1,20 @@
+/**
+Returns the type that is wrapped inside a `Promise` type.
+If the type is not a `Promise`, the type itself is returned.
+
+@example
+```
+import {PromiseValue} from 'type-fest';
+
+type AsyncData = Promise<string>;
+let asyncData: PromiseValue<AsyncData> = Promise.resolve('ABC');
+
+type Data = PromiseValue<AsyncData>;
+let data: Data = await asyncData;
+
+// Here's an example that shows how this type reacts to non-Promise types.
+type SyncData = PromiseValue<string>;
+let syncData: SyncData = getSyncData();
+```
+*/
+export type PromiseValue<PromiseType, Otherwise = PromiseType> = PromiseType extends Promise<infer Value> ? Value : Otherwise;

test-d/promise-value.ts

@@ -0,0 +1,14 @@
+import {expectType} from 'tsd';
+import {PromiseValue} from '..';
+
+type NumberPromise = Promise<number>;
+type Otherwise = object;
+
+// Test the normal behaviour.
+expectType<PromiseValue<NumberPromise>>(2);
+
+// Test what happens when the `PromiseValue` type is not handed a `Promise` type.
+expectType<PromiseValue<number>>(2);
+
+// Test the `Otherwise` generic parameter.
+expectType<PromiseValue<number, Otherwise>>({});