IterableElement: Improve docs and add tests for using it with Set (…

…#894)

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

readme.md

@@ -167,7 +167,7 @@ Click the type names for complete docs.
 - [`LiteralToPrimitive`](source/literal-to-primitive.d.ts) - Convert a [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types) to the [primitive type](source/primitive.d.ts) it belongs to.
 - [`LiteralToPrimitiveDeep`](source/literal-to-primitive-deep.d.ts) - Like `LiteralToPrimitive` except it converts literal types inside an object or array deeply.
 - [`Stringified`](source/stringified.d.ts) - Create a type with the keys of the given type changed to `string` type.
-- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.
+- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, `Array`, `Set`, `Map`, generator, stream, etc.
 - [`Entry`](source/entry.d.ts) - Create a type that represents the type of an entry of a collection.
 - [`Entries`](source/entries.d.ts) - Create a type that represents the type of the entries of a collection.
 - [`SetReturnType`](source/set-return-type.d.ts) - Create a function type with a return type of your choice and the same parameters as the given function type.
@@ -348,6 +348,9 @@ type ShouldBeNever = IfAny<'not any', 'not never', 'never'>;
 - `AllKeys` - See [`KeysOfUnion`](source/keys-of-union.d.ts)
 - `Branded` - See [`Tagged`](source/opaque.d.ts)
 - `Opaque` - See [`Tagged`](source/opaque.d.ts)
+- `SetElement` - See [`IterableElement`](source/iterable-element.d.ts)
+- `SetEntry` - See [`IterableElement`](source/iterable-element.d.ts)
+- `SetValues` - See [`IterableElement`](source/iterable-element.d.ts)
 
 ## Tips
 

source/iterable-element.d.ts

@@ -1,5 +1,5 @@
 /**
-Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.
+Get the element type of an `Iterable`/`AsyncIterable`. For example, `Array`, `Set`, `Map`, generator, stream, etc.
 
 This can be useful, for example, if you want to get the type that is yielded in a generator function. Often the return type of those functions are not specified.
 
@@ -33,7 +33,7 @@ async function * iAmGeneratorAsync() {
 type MeStringOrBoolean = IterableElement<ReturnType<typeof iAmGeneratorAsync>>
 ```
 
-Many types in JavaScript/TypeScript are iterables. This type works on all types that implement those interfaces. For example, `Array`, `Set`, `Map`, `stream.Readable`, etc.
+Many types in JavaScript/TypeScript are iterables. This type works on all types that implement those interfaces.
 
 An example with an array of strings:
 
@@ -44,6 +44,16 @@ import type {IterableElement} from 'type-fest';
 type MeString = IterableElement<string[]>
 ```
 
+@example
+```
+import type {IterableElement} from 'type-fest';
+
+const fruits = new Set(['🍎', '🍌', '🍉'] as const);
+
+type Fruit = IterableElement<typeof fruits>;
+//=> '🍎' | '🍌' | '🍉'
+```
+
 @category Iterable
 */
 export type IterableElement<TargetIterable> =

test-d/iterable-element.ts

@@ -1,4 +1,4 @@
-import {expectType} from 'tsd';
+import {expectAssignable, expectType} from 'tsd';
 import type {IterableElement} from '../index';
 
 declare const iterableElement: IterableElement<ReturnType<typeof secretGenerator>>;
@@ -16,3 +16,25 @@ async function * secretGeneratorAsync() {
 	yield true;
 	yield new Date();
 }
+
+const fruits = new Set(['🍎', '🍌', '🍉'] as const);
+
+type Fruit = IterableElement<typeof fruits>;
+
+expectAssignable<Fruit>('🍎');
+expectAssignable<Fruit>('🍌');
+expectAssignable<Fruit>('🍉');
+
+type VegetableSet = Set<'🥦' | '🥕' | '🌶'>;
+type Vegetable = IterableElement<VegetableSet>;
+
+expectAssignable<Vegetable>('🥦');
+expectAssignable<Vegetable>('🥕');
+expectAssignable<Vegetable>('🌶');
+
+type UserRolesSet = ReadonlySet<'regular' | 'contributor' | 'maintainer'>;
+type UserRole = IterableElement<UserRolesSet>;
+
+expectAssignable<UserRole>('regular');
+expectAssignable<UserRole>('contributor');
+expectAssignable<UserRole>('maintainer');