type-fest

import type {And} from 'type-fest';

And<true, true>;
//=> true

And<true, false>;
//=> false

import type {Arrayable} from 'type-fest';

function bundle(input: string, output: Arrayable<string>) {
	const outputList = Array.isArray(output) ? output : [output];

	// …

	for (const output of outputList) {
	  console.log(`write to: ${output}`);
	}
}

bundle('src/index.js', 'dist/index.js');
bundle('src/index.js', ['dist/index.cjs', 'dist/index.mjs']);

import type {ArrayIndices, ArrayValues} from 'type-fest';

const weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] as const;

type Weekday = ArrayIndices<typeof weekdays>;
type WeekdayName = ArrayValues<typeof weekdays>;

const getWeekdayName = (day: Weekday): WeekdayName => weekdays[day];

import type {ArraySlice} from 'type-fest';

type T0 = ArraySlice<[0, 1, 2, 3, 4]>;
//=> [0, 1, 2, 3, 4]

type T1 = ArraySlice<[0, 1, 2, 3, 4], 0, -1>;
//=> [0, 1, 2, 3]

type T2 = ArraySlice<[0, 1, 2, 3, 4], 1, -2>;
//=> [1, 2]

type T3 = ArraySlice<[0, 1, 2, 3, 4], -2, 4>;
//=> [3]

type T4 = ArraySlice<[0, 1, 2, 3, 4], -2, -1>;
//=> [3]

type T5 = ArraySlice<[0, 1, 2, 3, 4], 0, -999>;
//=> []

function arraySlice<
	const Array_ extends readonly unknown[],
	Start extends number = 0,
	End extends number = Array_['length'],
>(array: Array_, start?: Start, end?: End) {
	return array.slice(start, end) as ArraySlice<Array_, Start, End>;
}

const slice = arraySlice([1, '2', {a: 3}, [4, 5]], 0, -1);

typeof slice;
//=> [1, '2', { readonly a: 3; }]

slice[2].a;
//=> 3

// @ts-expect-error -- TS2493: Tuple type '[1, "2", {readonly a: 3}]' of length '3' has no element at index '3'.
slice[3];

type SomeMonths0 = ['January', 'April', 'June'];
type Mouths0 = ArraySplice<SomeMonths0, 1, 0, ['Feb', 'March']>;
//=> type Mouths0 = ['January', 'Feb', 'March', 'April', 'June'];

type SomeMonths1 = ['January', 'April', 'June'];
type Mouths1 = ArraySplice<SomeMonths1, 1, 1>;
//=> type Mouths1 = ['January', 'June'];

type SomeMonths2 = ['January', 'Foo', 'April'];
type Mouths2 = ArraySplice<SomeMonths2, 1, 1, ['Feb', 'March']>;
//=> type Mouths2 = ['January', 'Feb', 'March', 'April'];

import type {ArrayTail} from 'type-fest';

declare const curry: <Arguments extends unknown[], Return>(
	function_: (...arguments_: Arguments) => Return,
	...arguments_: ArrayTail<Arguments>
) => (...arguments_: ArrayTail<Arguments>) => Return;

const add = (a: number, b: number) => a + b;

const add3 = curry(add, 3);

add3(4);
//=> 7

import type {ArrayValues, ArrayIndices} from 'type-fest';

const weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] as const;

type WeekdayName = ArrayValues<typeof weekdays>;
type Weekday = ArrayIndices<typeof weekdays>;

const getWeekdayName = (day: Weekday): WeekdayName => weekdays[day];

import type {Asyncify} from 'type-fest';

// Synchronous function.
function getFooSync(someArg: SomeType): Foo {
	// …
}

type AsyncifiedFooGetter = Asyncify<typeof getFooSync>;
//=> type AsyncifiedFooGetter = (someArg: SomeType) => Promise<Foo>;

// Same as `getFooSync` but asynchronous.
const getFooAsync: AsyncifiedFooGetter = (someArg) => {
	// TypeScript now knows that `someArg` is `SomeType` automatically.
	// It also knows that this function must return `Promise<Foo>`.
	// If you have `@typescript-eslint/promise-function-async` linter rule enabled, it will even report that "Functions that return promises must be async.".

	// …
}

import type {AsyncReturnType} from 'type-fest';
import {asyncFunction} from 'api';

// This type resolves to the unwrapped return type of `asyncFunction`.
type Value = AsyncReturnType<typeof asyncFunction>;

async function doSomething(value: Value) {}

asyncFunction().then(value => doSomething(value));

import type {CamelCase} from 'type-fest';

// Simple

const someVariable: CamelCase<'foo-bar'> = 'fooBar';

// Advanced

type CamelCasedProperties<T> = {
	[K in keyof T as CamelCase<K>]: T[K]
};

interface RawOptions {
	'dry-run': boolean;
	'full_family_name': string;
	foo: number;
	BAR: string;
	QUZ_QUX: number;
	'OTHER-FIELD': boolean;
}

const dbResult: CamelCasedProperties<RawOptions> = {
	dryRun: true,
	fullFamilyName: 'bar.js',
	foo: 123,
	bar: 'foo',
	quzQux: 6,
	otherField: false
};

import type {CamelCasedProperties} from 'type-fest';

interface User {
	UserId: number;
	UserName: string;
}

const result: CamelCasedProperties<User> = {
	userId: 1,
	userName: 'Tom',
};

import type {CamelCasedPropertiesDeep} from 'type-fest';

interface User {
	UserId: number;
	UserName: string;
}

interface UserWithFriends {
	UserInfo: User;
	UserFriends: User[];
}

const result: CamelCasedPropertiesDeep<UserWithFriends> = {
	userInfo: {
		userId: 1,
		userName: 'Tom',
	},
	userFriends: [
		{
			userId: 2,
			userName: 'Jerry',
		},
		{
			userId: 3,
			userName: 'Spike',
		},
	],
};

import type {Primitive, ConditionalExcept} from 'type-fest';

class Awesome {
	name: string;
	successes: number;
	failures: bigint;

	run() {}
}

type ExceptPrimitivesFromAwesome = ConditionalExcept<Awesome, Primitive>;
//=> {run: () => void}

import type {ConditionalExcept} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c: () => void;
	d: {};
}

type NonStringKeysOnly = ConditionalExcept<Example, string>;
//=> {b: string | number; c: () => void; d: {}}

import type {ConditionalKeys} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c?: string;
	d: {};
}

type StringKeysOnly = ConditionalKeys<Example, string>;
//=> 'a'

import type {ConditionalKeys} from 'type-fest';

type StringKeysAndUndefined = ConditionalKeys<Example, string | undefined>;
//=> 'a' | 'c'

import type {Primitive, ConditionalPick} from 'type-fest';

class Awesome {
	name: string;
	successes: number;
	failures: bigint;

	run() {}
}

type PickPrimitivesFromAwesome = ConditionalPick<Awesome, Primitive>;
//=> {name: string; successes: number; failures: bigint}

import type {ConditionalPick} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c: () => void;
	d: {};
}

type StringKeysOnly = ConditionalPick<Example, string>;
//=> {a: string}

import type {ConditionalPickDeep} from 'type-fest';

interface Example {
	a: string;
	b: string | boolean;
	c: {
		d: string;
		e: {
			f?: string;
			g?: boolean;
			h: string | boolean;
			i: boolean | bigint;
		};
		j: boolean;
	};
}

type StringPick = ConditionalPickDeep<Example, string>;
//=> {a: string; c: {d: string}}

type StringPickOptional = ConditionalPickDeep<Example, string | undefined>;
//=> {a: string; c: {d: string; e: {f?: string}}}

type StringPickOptionalOnly = ConditionalPickDeep<Example, string | undefined, {condition: 'equality'}>;
//=> {c: {e: {f?: string}}}

type BooleanPick = ConditionalPickDeep<Example, boolean | undefined>;
//=> {c: {e: {g?: boolean}; j: boolean}}

type NumberPick = ConditionalPickDeep<Example, number>;
//=> {}

type StringOrBooleanPick = ConditionalPickDeep<Example, string | boolean>;
//=> {
// 	a: string;
// 	b: string | boolean;
// 	c: {
// 		d: string;
// 		e: {
// 			h: string | boolean
// 		};
// 		j: boolean;
// 	};
// }

type StringOrBooleanPickOnly = ConditionalPickDeep<Example, string | boolean, {condition: 'equality'}>;
//=> {b: string | boolean; c: {e: {h: string | boolean}}}

import type {DelimiterCase} from 'type-fest';

// Simple

const someVariable: DelimiterCase<'fooBar', '#'> = 'foo#bar';

// Advanced

type OddlyCasedProperties<T> = {
	[K in keyof T as DelimiterCase<K, '#'>]: T[K]
};

interface SomeOptions {
	dryRun: boolean;
	includeFile: string;
	foo: number;
}

const rawCliOptions: OddlyCasedProperties<SomeOptions> = {
	'dry#run': true,
	'include#file': 'bar.js',
	foo: 123
};

import type {DelimiterCasedProperties} from 'type-fest';

interface User {
	userId: number;
	userName: string;
}

const result: DelimiterCasedProperties<User, '-'> = {
	'user-id': 1,
	'user-name': 'Tom',
};

import type {DelimiterCasedPropertiesDeep} from 'type-fest';

interface User {
	userId: number;
	userName: string;
}

interface UserWithFriends {
	userInfo: User;
	userFriends: User[];
}

const result: DelimiterCasedPropertiesDeep<UserWithFriends, '-'> = {
	'user-info': {
	'user-id': 1,
		'user-name': 'Tom',
	},
	'user-friends': [
		{
			'user-id': 2,
			'user-name': 'Jerry',
		},
		{
			'user-id': 3,
			'user-name': 'Spike',
		},
	],
};

type A = {
	discriminant: 'A';
	foo: string;
	a: number;
};

type B = {
	discriminant: 'B';
	foo: string;
	b: string;
};

type Union = A | B;

type OmittedUnion = Omit<Union, 'foo'>;
//=> {discriminant: 'A' | 'B'}

const omittedUnion: OmittedUnion = createOmittedUnion();

if (omittedUnion.discriminant === 'A') {
	// We would like to narrow `omittedUnion`'s type
	// to `A` here, but we can't because `Omit`
	// doesn't distribute over unions.

	omittedUnion.a;
 	//=> Error: `a` is not a property of `{discriminant: 'A' | 'B'}`
}

type A = {
	discriminant: 'A';
	foo: string;
	a: number;
};

type B = {
	discriminant: 'B';
	foo: string;
	bar: string;
	b: string;
};

type C = {
	discriminant: 'C';
	bar: string;
	c: boolean;
};

// Notice that `foo` exists in `A` and `B`, but not in `C`, and
// `bar` exists in `B` and `C`, but not in `A`.

type Union = A | B | C;

type OmittedUnion = DistributedOmit<Union, 'foo' | 'bar'>;

const omittedUnion: OmittedUnion = createOmittedUnion();

if (omittedUnion.discriminant === 'A') {
	omittedUnion.a;
 	//=> OK

	omittedUnion.foo;
 	//=> Error: `foo` is not a property of `{discriminant: 'A'; a: string}`

	omittedUnion.bar;
 	//=> Error: `bar` is not a property of `{discriminant: 'A'; a: string}`
}

type A = {
	discriminant: 'A';
	foo: {
		bar: string;
	};
};

type B = {
	discriminant: 'B';
	foo: {
		baz: string;
	};
};

type Union = A | B;

type PickedUnion = Pick<Union, 'discriminant' | 'foo'>;
//=> {discriminant: 'A' | 'B', foo: {bar: string} | {baz: string}}

const pickedUnion: PickedUnion = createPickedUnion();

if (pickedUnion.discriminant === 'A') {
	// We would like to narrow `pickedUnion`'s type
	// to `A` here, but we can't because `Pick`
	// doesn't distribute over unions.

	pickedUnion.foo.bar;
	//=> Error: Property 'bar' does not exist on type '{bar: string} | {baz: string}'.
}

type A = {
	discriminant: 'A';
	foo: {
		bar: string;
	};
	extraneous: boolean;
};

type B = {
	discriminant: 'B';
	foo: {
		baz: string;
	};
	extraneous: boolean;
};

// Notice that `foo.bar` exists in `A` but not in `B`.

type Union = A | B;

type PickedUnion = DistributedPick<Union, 'discriminant' | 'foo'>;

const pickedUnion: PickedUnion = createPickedUnion();

if (pickedUnion.discriminant === 'A') {
	pickedUnion.foo.bar;
 	//=> OK

	pickedUnion.extraneous;
	//=> Error: Property `extraneous` does not exist on type `Pick<A, 'discriminant' | 'foo'>`.

	pickedUnion.foo.baz;
	//=> Error: `bar` is not a property of `{discriminant: 'A'; a: string}`.
}

import type {EmptyObject} from 'type-fest';

// The following illustrates the problem with `{}`.
const foo1: {} = {}; // Pass
const foo2: {} = []; // Pass
const foo3: {} = 42; // Pass
const foo4: {} = {a: 1}; // Pass

// With `EmptyObject` only the first case is valid.
const bar1: EmptyObject = {}; // Pass
const bar2: EmptyObject = 42; // Fail
const bar3: EmptyObject = []; // Fail
const bar4: EmptyObject = {a: 1}; // Fail

import type {Entries} from 'type-fest';

interface Example {
	someKey: number;
}

const manipulatesEntries = (examples: Entries<Example>) => examples.map(example => [
	// Does some arbitrary processing on the key (with type information available)
	example[0].toUpperCase(),

	// Does some arbitrary processing on the value (with type information available)
	example[1].toFixed()
]);

const example: Example = {someKey: 1};
const entries = Object.entries(example) as Entries<Example>;
const output = manipulatesEntries(entries);

// Objects
const objectExample = {a: 1};
const objectEntries: Entries<typeof objectExample> = [['a', 1]];

// Arrays
const arrayExample = ['a', 1];
const arrayEntries: Entries<typeof arrayExample> = [[0, 'a'], [1, 1]];

// Maps
const mapExample = new Map([['a', 1]]);
const mapEntries: Entries<typeof map> = [['a', 1]];

// Sets
const setExample = new Set(['a', 1]);
const setEntries: Entries<typeof setExample> = [['a', 'a'], [1, 1]];

import type {Entry} from 'type-fest';

interface Example {
	someKey: number;
}

const manipulatesEntry = (example: Entry<Example>) => [
	// Does some arbitrary processing on the key (with type information available)
	example[0].toUpperCase(),

	// Does some arbitrary processing on the value (with type information available)
	example[1].toFixed(),
];

const example: Example = {someKey: 1};
const entry = Object.entries(example)[0] as Entry<Example>;
const output = manipulatesEntry(entry);

// Objects
const objectExample = {a: 1};
const objectEntry: Entry<typeof objectExample> = ['a', 1];

// Arrays
const arrayExample = ['a', 1];
const arrayEntryString: Entry<typeof arrayExample> = [0, 'a'];
const arrayEntryNumber: Entry<typeof arrayExample> = [1, 1];

// Maps
const mapExample = new Map([['a', 1]]);
const mapEntry: Entry<typeof mapExample> = ['a', 1];

// Sets
const setExample = new Set(['a', 1]);
const setEntryString: Entry<typeof setExample> = ['a', 'a'];
const setEntryNumber: Entry<typeof setExample> = [1, 1];

type OnlyAcceptName = {name: string};

function onlyAcceptName(arguments_: OnlyAcceptName) {}

// TypeScript complains about excess properties when an object literal is provided.
onlyAcceptName({name: 'name', id: 1});
//=> `id` is excess

// TypeScript does not complain about excess properties when the provided value is a variable (not an object literal).
const invalidInput = {name: 'name', id: 1};
onlyAcceptName(invalidInput); // No errors

import {Exact} from 'type-fest';

type OnlyAcceptName = {name: string};

function onlyAcceptNameImproved<T extends Exact<OnlyAcceptName, T>>(arguments_: T) {}

const invalidInput = {name: 'name', id: 1};
onlyAcceptNameImproved(invalidInput); // Compilation error

import type {Except} from 'type-fest';

type Foo = {
	a: number;
	b: string;
};

type FooWithoutA = Except<Foo, 'a'>;
//=> {b: string}

const fooWithoutA: FooWithoutA = {a: 1, b: '2'};
//=> errors: 'a' does not exist in type '{ b: string; }'

type FooWithoutB = Except<Foo, 'b', {requireExactProps: true}>;
//=> {a: number} & Partial<Record<"b", never>>

const fooWithoutB: FooWithoutB = {a: 1, b: '2'};
//=> errors at 'b': Type 'string' is not assignable to type 'undefined'.

import type {FindGlobalInstanceType} from 'type-fest';

class Point {
	constructor(public x: number, public y: number) {}
}

type PointLike = Point | FindGlobalInstanceType<'DOMPoint'>;

import type {FindGlobalInstanceType} from 'type-fest';

declare global {
	// Class syntax won't add the key to `globalThis`
	class Foo {}

	// interface + constructor style works
	interface Bar {}
	var Bar: new () => Bar; // Not let or const
}

type FindFoo = FindGlobalInstanceType<'Foo'>; // Doesn't work
type FindBar = FindGlobalInstanceType<'Bar'>; // Works

import type {FindGlobalType} from 'type-fest';

declare global {
	const foo: number; // let and const don't work
	var bar: string;   // var works
}

type FooType = FindGlobalType<'foo'>     //=> never (let/const don't work)
type BarType = FindGlobalType<'bar'>     //=> string
type OtherType = FindGlobalType<'other'> //=> never (no global named 'other')

import type {Finite} from 'type-fest';

declare function setScore<T extends number>(length: Finite<T>): void;

import type {FixedLengthArray} from 'type-fest';

type FencingTeam = FixedLengthArray<string, 3>;

const guestFencingTeam: FencingTeam = ['Josh', 'Michael', 'Robert'];

const homeFencingTeam: FencingTeam = ['George', 'John'];
//=> error TS2322: Type string[] is not assignable to type 'FencingTeam'

guestFencingTeam.push('Sam');
//=> error TS2339: Property 'push' does not exist on type 'FencingTeam'

import type {Float} from 'type-fest';

declare function setPercentage<T extends number>(length: Float<T>): void;

import type {Get} from 'type-fest';
import * as lodash from 'lodash';

const get = <BaseType, Path extends string | readonly string[]>(object: BaseType, path: Path): Get<BaseType, Path> =>
	lodash.get(object, path);

interface ApiResponse {
	hits: {
		hits: Array<{
			_id: string
			_source: {
				name: Array<{
					given: string[]
					family: string
				}>
				birthDate: string
			}
		}>
	}
}

const getName = (apiResponse: ApiResponse) =>
	get(apiResponse, 'hits.hits[0]._source.name');
	//=> Array<{given: string[]; family: string}> | undefined

// Path also supports a readonly array of strings
const getNameWithPathArray = (apiResponse: ApiResponse) =>
	get(apiResponse, ['hits','hits', '0', '_source', 'name'] as const);
	//=> Array<{given: string[]; family: string}> | undefined

// Non-strict mode:
Get<string[], '3', {strict: false}> //=> string
Get<Record<string, string>, 'foo', {strict: true}> // => string

import type {Tagged} from 'type-fest';

type JsonOf<T> = Tagged<string, 'JSON', T>;

function stringify<T>(it: T) {
  return JSON.stringify(it) as JsonOf<T>;
}

function parse<T extends JsonOf<unknown>>(it: T) {
  return JSON.parse(it) as GetTagMetadata<T, 'JSON'>;
}

const x = stringify({ hello: 'world' });
const parsed = parse(x); // The type of `parsed` is { hello: string }

import type {GlobalThis} from 'type-fest';

type ExtraGlobals = GlobalThis & {
	readonly GLOBAL_TOKEN: string;
};

(globalThis as ExtraGlobals).GLOBAL_TOKEN;

import type {GreaterThan} from 'type-fest';

GreaterThan<1, -5>;
//=> true

GreaterThan<1, 1>;
//=> false

GreaterThan<1, 5>;
//=> false

import type {GreaterThanOrEqual} from 'type-fest';

GreaterThanOrEqual<1, -5>;
//=> true

GreaterThanOrEqual<1, 1>;
//=> true

GreaterThanOrEqual<1, 5>;
//=> false

import type {HasOptionalKeys, OptionalKeysOf} from 'type-fest';

type UpdateService<Entity extends object> = {
	removeField: HasOptionalKeys<Entity> extends true
		? (field: OptionalKeysOf<Entity>) => Promise<void>
		: never
}

import type {HasReadonlyKeys, ReadonlyKeysOf} from 'type-fest';

type UpdateService<Entity extends object> = {
	removeField: HasReadonlyKeys<Entity> extends true
		? (field: ReadonlyKeysOf<Entity>) => Promise<void>
		: never
}

import type {HasRequiredKeys} from 'type-fest';

type GeneratorOptions<Template extends object> = {
	prop1: number;
	prop2: string;
} & (HasRequiredKeys<Template> extends true
	? {template: Template}
	: {template?: Template});

interface Template1 {
	optionalSubParam?: string;
}

interface Template2 {
	requiredSubParam: string;
}

type Options1 = GeneratorOptions<Template1>;
type Options2 = GeneratorOptions<Template2>;

const optA: Options1 = {
	prop1: 0,
	prop2: 'hi'
};
const optB: Options1 = {
	prop1: 0,
	prop2: 'hi',
	template: {}
};
const optC: Options1 = {
	prop1: 0,
	prop2: 'hi',
	template: {
		optionalSubParam: 'optional value'
	}
};

const optD: Options2 = {
	prop1: 0,
	prop2: 'hi',
	template: {
		requiredSubParam: 'required value'
	}
};

import type {HasWritableKeys, WritableKeysOf} from 'type-fest';

type UpdateService<Entity extends object> = {
	removeField: HasWritableKeys<Entity> extends true
		? (field: WritableKeysOf<Entity>) => Promise<void>
		: never
}

import type {IfAny} from 'type-fest';

type ShouldBeTrue = IfAny<any>;
//=> true

type ShouldBeBar = IfAny<'not any', 'foo', 'bar'>;
//=> 'bar'

import type {IfEmptyObject} from 'type-fest';

type ShouldBeTrue = IfEmptyObject<{}>;
//=> true

type ShouldBeBar = IfEmptyObject<{key: any}, 'foo', 'bar'>;
//=> 'bar'

import type {IfNever} from 'type-fest';

type ShouldBeTrue = IfNever<never>;
//=> true

type ShouldBeBar = IfNever<'not never', 'foo', 'bar'>;
//=> 'bar'

import type {IfNull} from 'type-fest';

type ShouldBeTrue = IfNull<null>;
//=> true

type ShouldBeBar = IfNull<'not null', 'foo', 'bar'>;
//=> 'bar'

import type {IfUnknown} from 'type-fest';

type ShouldBeTrue = IfUnknown<unknown>;
//=> true

type ShouldBeBar = IfUnknown<'not unknown', 'foo', 'bar'>;
//=> 'bar'

import type {Includes} from 'type-fest';

type hasRed<array extends any[]> = Includes<array, 'red'>;

type Integer = Integer<1>;
//=> 1

type IntegerWithDecimal = Integer<1.0>;
//=> 1

type NegativeInteger = Integer<-1>;
//=> -1

type Float = Integer<1.5>;
//=> never

// Supports non-decimal numbers

type OctalInteger: Integer<0o10>;
//=> 0o10

type BinaryInteger: Integer<0b10>;
//=> 0b10

type HexadecimalInteger: Integer<0x10>;
//=> 0x10

import type {Integer} from 'type-fest';

declare function setYear<T extends number>(length: Integer<T>): void;

import type {IntRange} from 'type-fest';

// Create union type `0 | 1 | ... | 9`
type ZeroToNine = IntRange<0, 10>;

// Create union type `100 | 200 | 300 | ... | 900`
type Hundreds = IntRange<100, 901, 100>;

import type {InvariantOf} from 'type-fest';

class Animal {
	constructor(public name: string){}
}

class Cat extends Animal {
	meow() {}
}

let animalArray: Animal[] = [animal];
let catArray: Cat[] = [cat];

animalArray = catArray; // Okay if covariant
animalArray.push(new Animal('another animal')); // Pushed an animal into catArray
catArray.forEach(c => c.meow()); // Allowed but, error at runtime

let invariantAnimalArray: InvariantOf<Animal>[] = [animal] as InvariantOf<Animal>[];
let invariantCatArray: InvariantOf<Cat>[] = [cat] as InvariantOf<Cat>[];

invariantAnimalArray = invariantCatArray; // Error: Type 'InvariantOf<Cat>[]' is not assignable to type 'InvariantOf<Animal>[]'.

import type {InvariantOf} from 'type-fest';

// In covariance (default)

interface FooBar {
	foo: number;
	bar: string
}

interface FooBarBaz extends FooBar {
	baz: boolean
}

declare const fooBar: FooBar
declare const fooBarBaz: FooBarBaz

function keyOfFooBar(fooBar: FooBar) {
	return Object.keys(fooBar) as (keyof FooBar)[]
}

keyOfFooBar(fooBar) //=> (keyof FooBar)[]
keyOfFooBar(fooBarBaz) //=> (keyof FooBar)[] but, (keyof FooBarBaz)[] at runtime

// In invariance

export function invariantOf<Type>(value: Type): InvariantOf<Type> {
	return value as InvariantOf<Type>;
}

function keyOfInvariantFooBar(fooBar: InvariantOf<FooBar>) {
	return Object.keys(fooBar) as (keyof FooBar)[]
}

keyOfInvariantFooBar(invariantOf(fooBar)); // (keyof FooBar)[]
keyOfInvariantFooBar(invariantOf(fooBarBaz)); // Error: Argument of type 'InvariantOf<FooBarBaz>' is not assignable to parameter of type 'InvariantOf<FooBar>'.