Add ValueOf type (#81)

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 {ValueOf} from './source/value-of';
 export {PromiseValue} from './source/promise-value';
 export {AsyncReturnType} from './source/async-return-type';
 export {ConditionalExcept} from './source/conditional-except';

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.
+- [`ValueOf`](source/value-of.d.ts) - Create a union of the given object's values, and optionally specify which keys to get the values from.
 - [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise`.
 - [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`.
 - [`ConditionalKeys`](source/conditional-keys.d.ts) - Extract keys from a shape where values extend the given `Condition` type.

source/value-of.d.ts

@@ -0,0 +1,40 @@
+/**
+Create a union of the given object's values, and optionally specify which keys to get the values from.
+
+Please upvote [this issue](https://github.com/microsoft/TypeScript/issues/31438) if you want to have this type as a built-in in TypeScript.
+
+@example
+```
+// data.json
+{
+	'foo': 1,
+	'bar': 2,
+	'biz': 3
+}
+
+// main.ts
+import {ValueOf} from 'type-fest';
+import data = require('./data.json');
+
+export function getData(name: string): ValueOf<typeof data> {
+	return data[name];
+}
+
+export function onlyBar(name: string): ValueOf<typeof data, 'bar'> {
+	return data[name];
+}
+
+// file.ts
+import {getData, onlyBar} from './main';
+
+getData('foo');
+//=> 1
+
+onlyBar('foo');
+//=> TypeError ...
+
+onlyBar('bar');
+//=> 2
+```
+*/
+export type ValueOf<ObjectType, ValueType extends keyof ObjectType = keyof ObjectType> = ObjectType[ValueType];

test-d/value-of.ts

@@ -0,0 +1,12 @@
+import {expectType, expectError} from 'tsd';
+import {ValueOf} from '..';
+
+const value: ValueOf<{a: 1; b: 2; c: 3}> = 3;
+const valueRestricted: ValueOf<{a: 1; b: 2; c: 3}, 'a'> = 1;
+
+expectType<1 | 2 | 3>(value);
+expectError<4>(value);
+
+expectType<1>(valueRestricted);
+expectError<2>(valueRestricted);
+expectError<4>(valueRestricted);