From 7fe70fd821e67a1273c5beea227bca3a677dd8be Mon Sep 17 00:00:00 2001
From: Resynth <49915996+resynth1943@users.noreply.github.com>
Date: Thu, 6 Feb 2020 20:24:28 +0000
Subject: [PATCH] Add `PromiseValue` type (#75)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
---
 index.d.ts                |  1 +
 readme.md                 |  1 +
 source/promise-value.d.ts | 20 ++++++++++++++++++++
 test-d/promise-value.ts   | 14 ++++++++++++++
 4 files changed, 36 insertions(+)
 create mode 100644 source/promise-value.d.ts
 create mode 100644 test-d/promise-value.ts

diff --git a/index.d.ts b/index.d.ts
index d611efb9f..7ade21dfc 100644
--- a/index.d.ts
+++ b/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';
diff --git a/readme.md b/readme.md
index fd8907a1c..c82ea310f 100644
--- a/readme.md
+++ b/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
 
diff --git a/source/promise-value.d.ts b/source/promise-value.d.ts
new file mode 100644
index 000000000..7686dd11d
--- /dev/null
+++ b/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;
diff --git a/test-d/promise-value.ts b/test-d/promise-value.ts
new file mode 100644
index 000000000..d055c6c7e
--- /dev/null
+++ b/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>>({});