type-fest 5.x

Complete instructions

Verified against type-fest@5.9.0 on 2026-09-08. 3 of 3 examples executed.

type-fest v5

type-fest v5.0.0 is a type-only package. It exports TypeScript declarations from the package root; it does not provide runtime functions. Use it from TypeScript with TypeScript >=5.9, {strict: true}, and an ESM setup. The package declares Node >=20 and is pure ESM.

Importing

Use type-only imports from the package root:

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

The package has no documented runtime API. Do not write a plain-script example that expects an imported function, constructor, or runtime value. TypeScript's compiler or type-aware tooling must consume the declarations. The examples below are standalone Bun scripts: their runtime assertions only verify that the type-only code erases cleanly, while the assignments demonstrate the intended compile-time types.

Except

Except<ObjectType, KeysType, Options> removes selected keys from an object type. KeysType must extend keyof ObjectType.

import assert from 'node:assert/strict';
import type {Except} from 'type-fest';

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

type FooWithoutA = Except<Foo, 'a'>;

const value: FooWithoutA = {b: '2'};
assert.deepEqual(value, {b: '2'});

The older, commonly shown usage shape remains valid in v5:

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

type Foo = {unicorn: string; rainbow: boolean};
type FooWithoutRainbow = Except<Foo, 'rainbow'>;

Do not treat Except as a runtime object-filtering function. It changes only the static type; it does not remove properties from a JavaScript value.

Tagged types

Tagged<Type, TagName, TagMetadata> creates a branded type while retaining the underlying Type. Different tags prevent accidental interchangeability even when their underlying types are identical.

import assert from 'node:assert/strict';
import type {Tagged} from 'type-fest';

type AccountNumber = Tagged<number, 'AccountNumber'>;
type AccountBalance = Tagged<number, 'AccountBalance'>;

function createAccountNumber(): AccountNumber {
	return 2 as AccountNumber;
}

function getMoneyFor(accountNumber: AccountNumber): AccountBalance {
	assert.equal(accountNumber, 2);
	return 4 as AccountBalance;
}

const accountNumber = createAccountNumber();
const balance = getMoneyFor(accountNumber);
assert.equal(balance, 4);
assert.equal(accountNumber + 2, 4);

The tag is compile-time-only. Arithmetic and other operations on the underlying value remain possible, but an untagged number or a differently tagged number is not accepted where AccountNumber is required.

Metadata, GetTagMetadata, and UnwrapTagged

A tag can carry metadata as its third parameter. GetTagMetadata<Type, TagName> retrieves metadata for a tag, and UnwrapTagged<TaggedType> removes all tags and returns the underlying type.

import assert from 'node:assert/strict';
import type {GetTagMetadata, Tagged, UnwrapTagged} from 'type-fest';

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

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

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

const encoded = stringify({hello: 'world'});
const parsed: {hello: string} = parse(encoded);
assert.deepEqual(parsed, {hello: 'world'});

const plain: UnwrapTagged<typeof encoded> = 'encoded JSON';
assert.equal(plain, 'encoded JSON');

Tagged does not serialize a tag or metadata into the value. The JSON example uses casts at the serialization boundary because the type information is represented only in TypeScript.

Other exports

The v5 root re-exports many declaration-only utilities, including PackageJson, ArraySlice, CamelCase, Jsonify, Merge, Paths, and SetReturnType. They are types rather than runtime helpers. This skill does not infer their exact parameters or behavior beyond the researched surface above.

v5 migration traps

• v5 is pure ESM and declares "type": "module". It requires Node >=20 and TypeScript >=5.9; configure TypeScript with {strict: true}.
• The v4 name StringKeyOf was renamed to KeyAsString in v5:

  // v4
  import type {StringKeyOf} from 'type-fest';
  
  // v5
  import type {KeyAsString} from 'type-fest';
  

• ObservableLike moved to the ./globals sub-export. The package also documents the If* types as deprecated in favor of If.
• ArrayTail preserves readonly by default in v5 and no longer uses preserveReadonly.
• Naming-case types disable preserveConsecutiveUppercase by default.
• PartialDeep disables allowUndefinedInNonTupleArrays by default.
• Split enables strictLiteralChecks by default.
• Paths defaults maxRecursionDepth to 5, rather than v4's 10.

Not covered

This skill does not cover the exact signatures or usage of the other root exports, the ./globals declarations beyond the ObservableLike move, compiler configuration beyond the researched v5 requirements, or behavior of any runtime tool. type-fest itself exposes no documented runtime API; the remaining surface must be checked through TypeScript's compiler or type-aware tooling.