Fix CI

Fixes #220

.github/funding.yml

@@ -1,2 +0,0 @@
-github: sindresorhus
-tidelift: npm/@sindresorhus/is

.github/workflows/main.yml

@@ -0,0 +1,21 @@
+name: CI
+on:
+  - push
+  - pull_request
+jobs:
+  test:
+    name: Node.js ${{ matrix.node-version }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version:
+          - 24
+          - 22
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-node@v6
+        with:
+          node-version: ${{ matrix.node-version }}
+      - run: npm install
+      - run: npm test

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 yarn.lock
-dist
+/distribution
+.tsimp

.travis.yml

@@ -1,5 +0,0 @@
-language: node_js
-node_js:
-  - '14'
-  - '12'
-  - '10'

AGENTS.md

@@ -0,0 +1,9 @@
+# Notes
+
+## Branded types for type guards
+
+TypeScript type guards narrow in both branches. If `is.integer(n)` returns `value is number` and the input is `number`, the false branch computes `Exclude<number, number>` = `never`. This makes common patterns like `if (!is.integer(n)) throw; use(n)` fail because `n` becomes `never` after the guard.
+
+To avoid this, type guard predicates use branded types (e.g., `number & {readonly __brand: 'Integer'}`, `string & {readonly __brand: 'UrlString'}`). A branded subtype ensures the false branch stays the original type (e.g., `Exclude<number, Integer>` = `number`).
+
+Assert functions (`asserts value is T`) don't need branded types since they throw on failure and have no false branch. They use plain types like `asserts value is number`.

CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "@sindresorhus/is",
-	"version": "3.0.0",
+	"version": "8.1.0",
 	"description": "Type check values",
 	"license": "MIT",
 	"repository": "sindresorhus/is",
@@ -10,17 +10,22 @@
 		"email": "sindresorhus@gmail.com",
 		"url": "https://sindresorhus.com"
 	},
-	"main": "dist",
+	"type": "module",
+	"exports": {
+		"types": "./distribution/index.d.ts",
+		"default": "./distribution/index.js"
+	},
+	"sideEffects": false,
 	"engines": {
-		"node": ">=10"
+		"node": ">=22"
 	},
 	"scripts": {
-		"build": "del dist && tsc",
-		"test": "xo && ava",
-		"prepublishOnly": "npm run build"
+		"build": "del distribution && tsc",
+		"test": "tsc --noEmit && tsc --project test/tsconfig.json --noEmit && xo && node --experimental-transform-types --test test/test.ts",
+		"prepare": "npm run build"
 	},
 	"files": [
-		"dist"
+		"distribution"
 	],
 	"keywords": [
 		"type",
@@ -46,51 +51,26 @@
 		"typeguards",
 		"types"
 	],
-	"devDependencies": {
-		"@sindresorhus/tsconfig": "^0.7.0",
-		"@types/jsdom": "^16.1.0",
-		"@types/node": "^14.0.13",
-		"@types/zen-observable": "^0.8.0",
-		"@typescript-eslint/eslint-plugin": "^2.20.0",
-		"@typescript-eslint/parser": "^2.20.0",
-		"ava": "^3.3.0",
-		"del-cli": "^2.0.0",
-		"eslint-config-xo-typescript": "^0.26.0",
-		"jsdom": "^16.0.1",
-		"rxjs": "^6.4.0",
-		"tempy": "^0.4.0",
-		"ts-node": "^8.3.0",
-		"typescript": "~3.8.2",
-		"xo": "^0.26.1",
-		"zen-observable": "^0.8.8"
-	},
-	"types": "dist",
-	"sideEffects": false,
-	"ava": {
-		"extensions": [
-			"ts"
-		],
-		"require": [
-			"ts-node/register"
-		]
-	},
 	"xo": {
-		"extends": "xo-typescript",
-		"extensions": [
-			"ts"
-		],
-		"parserOptions": {
-			"project": "./tsconfig.xo.json"
-		},
-		"globals": [
-			"BigInt",
-			"BigInt64Array",
-			"BigUint64Array"
-		],
 		"rules": {
-			"@typescript-eslint/promise-function-async": "off",
-			"@typescript-eslint/no-empty-function": "off",
-			"@typescript-eslint/explicit-function-return-type": "off"
+			"@typescript-eslint/no-unsafe-enum-comparison": "off",
+			"@typescript-eslint/no-confusing-void-expression": "off",
+			"@typescript-eslint/no-unsafe-type-assertion": "off",
+			"@stylistic/operator-linebreak": "off"
 		}
+	},
+	"devDependencies": {
+		"@sindresorhus/tsconfig": "^8.1.0",
+		"@types/jsdom": "^28.0.1",
+		"@types/node": "^25.5.2",
+		"@types/zen-observable": "^0.8.7",
+		"del-cli": "^7.0.0",
+		"expect-type": "^1.3.0",
+		"jsdom": "^29.0.2",
+		"rxjs": "^7.8.2",
+		"tempy": "^3.2.0",
+		"typescript": "6.0.2",
+		"xo": "^2.0.2",
+		"zen-observable": "^0.10.0"
 	}
 }

readme.md

@@ -1,4 +1,4 @@
-# is [![Build Status](https://travis-ci.org/sindresorhus/is.svg?branch=master)](https://travis-ci.org/sindresorhus/is)
+# is
 
 > Type check values
 
@@ -17,14 +17,14 @@ For example, `is.string('🦄') //=> true`
 
 ## Install
 
-```
-$ npm install @sindresorhus/is
+```sh
+npm install @sindresorhus/is
 ```
 
 ## Usage
 
 ```js
-const is = require('@sindresorhus/is');
+import is from '@sindresorhus/is';
 
 is('🦄');
 //=> 'string'
@@ -39,12 +39,21 @@ is.number(6);
 [Assertions](#type-assertions) perform the same type checks, but throw an error if the type does not match.
 
 ```js
-const {assert} = require('@sindresorhus/is');
+import {assert} from '@sindresorhus/is';
 
 assert.string(2);
 //=> Error: Expected value which is `string`, received value of type `number`.
 ```
 
+Assertions (except `assertAll` and `assertAny`) also support an optional custom error message.
+
+```js
+import {assert} from '@sindresorhus/is';
+
+assert.nonEmptyString(process.env.API_URL, 'The API_URL environment variable is required.');
+//=> Error: The API_URL environment variable is required.
+```
+
 And with TypeScript:
 
 ```ts
@@ -54,6 +63,18 @@ assert.string(foo);
 // `foo` is now typed as a `string`.
 ```
 
+### Named exports
+
+Named exports allow tooling to perform tree-shaking, potentially reducing bundle size by including only code from the methods that are used.
+
+Every method listed below is available as a named export. Each method is prefixed by either `is` or `assert` depending on usage.
+
+For example:
+
+```js
+import {assertNull, isUndefined} from '@sindresorhus/is';
+```
+
 ## API
 
 ### is(value)
@@ -72,16 +93,23 @@ Example:
 - `'Function'`
 - `'Object'`
 
+This method is also exported as `detect`. You can import it like this:
+
+```js
+import {detect} from '@sindresorhus/is';
+```
+
 Note: It will throw an error if you try to feed it object-wrapped primitives, as that's a bad practice. For example `new String('foo')`.
 
 ### is.{method}
 
-All the below methods accept a value and returns a boolean for whether the value is of the desired type.
+All the below methods accept a value and return a boolean for whether the value is of the desired type.
 
 #### Primitives
 
 ##### .undefined(value)
 ##### .null(value)
+
 ##### .string(value)
 ##### .number(value)
 
@@ -93,9 +121,34 @@ Note: `is.number(NaN)` returns `false`. This intentionally deviates from `typeof
 
 #### Built-in types
 
-##### .array(value)
+##### .array(value, assertion?)
+
+Returns true if `value` is an array and all of its items match the assertion (if provided).
+
+```js
+is.array(value); // Validate `value` is an array.
+is.array(value, is.number); // Validate `value` is an array and all of its items are numbers.
+```
+
+##### .arrayOf(predicate)
+
+Returns a type guard that checks if `value` is an array where every item matches the predicate. Useful for composing with other methods.
+
+```js
+const isStringArray = is.arrayOf(is.string);
+
+isStringArray(['a', 'b']); //=> true
+isStringArray(['a', 1]); //=> false
+```
+
 ##### .function(value)
+
 ##### .buffer(value)
+
+> [!NOTE]
+> [Prefer using `Uint8Array` instead of `Buffer`.](https://sindresorhus.com/blog/goodbye-nodejs-buffer)
+
+##### .blob(value)
 ##### .object(value)
 
 Keep in mind that [functions are objects too](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions).
@@ -183,6 +236,7 @@ is.boundFunction(function () {});
 ##### .set(value)
 ##### .weakMap(value)
 ##### .weakSet(value)
+##### .weakRef(value)
 
 #### Typed arrays
 
@@ -204,19 +258,47 @@ is.boundFunction(function () {});
 ##### .sharedArrayBuffer(value)
 ##### .dataView(value)
 
+##### .enumCase(value, enum)
+
+TypeScript-only. Returns `true` if `value` is a member of `enum`.
+
+```ts
+enum Direction {
+	Ascending = 'ascending',
+	Descending = 'descending'
+}
+
+is.enumCase('ascending', Direction);
+//=> true
+
+is.enumCase('other', Direction);
+//=> false
+```
+
 #### Emptiness
 
 ##### .emptyString(value)
 
 Returns `true` if the value is a `string` and the `.length` is 0.
 
+##### .emptyStringOrWhitespace(value)
+
+Returns `true` if `is.emptyString(value)` or if it's a `string` that is all whitespace.
+
 ##### .nonEmptyString(value)
 
 Returns `true` if the value is a `string` and the `.length` is more than 0.
 
-##### .emptyStringOrWhitespace(value)
+##### .nonEmptyStringAndNotWhitespace(value)
 
-Returns `true` if `is.emptyString(value)` or if it's a `string` that is all whitespace.
+Returns `true` if the value is a `string` that is not empty and not whitespace.
+
+```js
+const values = ['property1', '', null, 'property2', '    ', undefined];
+
+values.filter(is.nonEmptyStringAndNotWhitespace);
+//=> ['property1', 'property2']
+```
 
 ##### .emptyArray(value)
 
@@ -329,7 +411,15 @@ Returns `true` if `value` is one of: `false`, `0`, `''`, `null`, `undefined`, `N
 ##### .nullOrUndefined(value)
 ##### .primitive(value)
 
-JavaScript primitives are as follows: `null`, `undefined`, `string`, `number`, `boolean`, `symbol`.
+JavaScript primitives are as follows:
+
+- `null`
+- `undefined`
+- `string`
+- `number`
+- `boolean`
+- `symbol`
+- `bigint`
 
 ##### .integer(value)
 
@@ -345,7 +435,7 @@ An object is plain if it's created by either `{}`, `new Object()`, or `Object.cr
 ##### .asyncIterable(value)
 ##### .class(value)
 
-Returns `true` for instances created by a class.
+Returns `true` if the value is a class constructor.
 
 ##### .typedArray(value)
 
@@ -364,6 +454,54 @@ function foo() {
 foo();
 ```
 
+##### .tupleLike(value, guards)
+
+A `value` is tuple-like if it matches the provided `guards` array both in `.length` and in types.
+
+```js
+is.tupleLike([1], [is.number]);
+//=> true
+```
+
+```js
+function foo() {
+	const tuple = [1, '2', true];
+	if (is.tupleLike(tuple, [is.number, is.string, is.boolean])) {
+		tuple // [number, string, boolean]
+	}
+}
+
+foo();
+```
+
+##### .finiteNumber(value)
+
+Check if `value` is a number and is finite. Excludes `Infinity` and `-Infinity`.
+
+##### .positiveNumber(value)
+
+Check if `value` is a number and is more than 0.
+
+##### .negativeNumber(value)
+
+Check if `value` is a number and is less than 0.
+
+##### .nonNegativeNumber(value)
+
+Check if `value` is a number and is 0 or more.
+
+##### .positiveInteger(value)
+
+Check if `value` is an integer and is more than 0.
+
+##### .negativeInteger(value)
+
+Check if `value` is an integer and is less than 0.
+
+##### .nonNegativeInteger(value)
+
+Check if `value` is an integer and is 0 or more.
+
 ##### .inRange(value, range)
 
 Check if `value` (number) is in the given `range`. The range is an array of two values, lower bound and upper bound, in no specific order.
@@ -382,16 +520,16 @@ Check if `value` (number) is in the range of `0` to `upperBound`.
 is.inRange(3, 10);
 ```
 
-##### .domElement(value)
+##### .htmlElement(value)
 
-Returns `true` if `value` is a DOM Element.
+Returns `true` if `value` is an [HTMLElement](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement).
 
 ##### .nodeStream(value)
 
 Returns `true` if `value` is a Node.js [stream](https://nodejs.org/api/stream.html).
 
 ```js
-const fs = require('fs');
+import fs from 'node:fs';
 
 is.nodeStream(fs.createReadStream('unicorn.png'));
 //=> true
@@ -402,7 +540,7 @@ is.nodeStream(fs.createReadStream('unicorn.png'));
 Returns `true` if `value` is an `Observable`.
 
 ```js
-const {Observable} = require('rxjs');
+import {Observable} from 'rxjs';
 
 is.observable(new Observable());
 //=> true
@@ -420,6 +558,32 @@ Returns `true` if `value` is an even integer.
 
 Returns `true` if `value` is an odd integer.
 
+##### .propertyKey(value)
+
+Returns `true` if `value` can be used as an object property key (either `string`, `number`, or `symbol`).
+
+##### .formData(value)
+
+Returns `true` if `value` is an instance of the [`FormData` class](https://developer.mozilla.org/en-US/docs/Web/API/FormData).
+
+```js
+const data = new FormData();
+
+is.formData(data);
+//=> true
+```
+
+##### .urlSearchParams(value)
+
+Returns `true` if `value` is an instance of the [`URLSearchParams` class](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams).
+
+```js
+const searchParams = new URLSearchParams();
+
+is.urlSearchParams(searchParams);
+//=> true
+```
+
 ##### .any(predicate | predicate[], ...values)
 
 Using a single `predicate` argument, returns `true` if **any** of the input `values` returns true in the `predicate`:
@@ -442,6 +606,31 @@ is.any([is.boolean, is.number], 'unicorns', [], new Map());
 //=> false
 ```
 
+##### .any(predicate[])
+
+Using an array of `predicate[]` without values, returns a combined type guard that checks if a value matches **any** of the predicates:
+
+```js
+const isStringOrNumber = is.any([is.string, is.number]);
+
+isStringOrNumber('hello');
+//=> true
+
+isStringOrNumber(123);
+//=> true
+
+isStringOrNumber(true);
+//=> false
+```
+
+This is useful for composing with other methods like `is.optional`:
+
+```js
+is.optional(value, is.any([is.string, is.number]));
+```
+
+An empty predicate array currently returns a predicate that always returns `false`. This will throw in the next major release.
+
 ##### .all(predicate, ...values)
 
 Returns `true` if **all** of the input `values` returns true in the `predicate`:
@@ -454,6 +643,94 @@ is.all(is.string, '🦄', [], 'unicorns');
 //=> false
 ```
 
+##### .all(predicate[])
+
+Using an array of `predicate[]` without values, returns a combined type guard that checks if a value matches **all** of the predicates:
+
+```js
+const isArrayAndNonEmpty = is.all([is.array, is.nonEmptyArray]);
+
+isArrayAndNonEmpty(['hello']);
+//=> true
+
+isArrayAndNonEmpty([]);
+//=> false
+```
+
+This is useful for composing with other methods like `is.optional`:
+
+```js
+is.optional(value, is.all([is.object, is.plainObject]));
+```
+
+An empty predicate array currently returns a predicate that always returns `true`. This will throw in the next major release.
+
+##### .optional(value, predicate)
+
+Returns `true` if `value` is `undefined` or satisfies the given `predicate`.
+
+```js
+is.optional(undefined, is.string);
+//=> true
+
+is.optional('🦄', is.string);
+//=> true
+
+is.optional(123, is.string);
+//=> false
+```
+
+##### .oneOf(values)
+
+Returns a type guard that checks if `value` is one of the given `values`. Best used with `as const` for precise type narrowing.
+
+```ts
+const isDirection = is.oneOf(['north', 'south', 'east', 'west'] as const);
+
+isDirection('north'); //=> true
+isDirection('up'); //=> false
+```
+
+##### .validDate(value)
+
+Returns `true` if the value is a valid date.
+
+All [`Date`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date) objects have an internal timestamp value which is the number of milliseconds since the [Unix epoch](https://developer.mozilla.org/en-US/docs/Glossary/Unix_time). When a new `Date` is constructed with bad inputs, no error is thrown. Instead, a new `Date` object is returned. But the internal timestamp value is set to `NaN`, which is an `'Invalid Date'`. Bad inputs can be an non-parsable date string, a non-numeric value or a number that is outside of the expected range for a date value.
+
+```js
+const valid = new Date('2000-01-01');
+
+is.date(valid);
+//=> true
+valid.getTime();
+//=> 946684800000
+valid.toUTCString();
+//=> 'Sat, 01 Jan 2000 00:00:00 GMT'
+is.validDate(valid);
+//=> true
+
+const invalid = new Date('Not a parsable date string');
+
+is.date(invalid);
+//=> true
+invalid.getTime();
+//=> NaN
+invalid.toUTCString();
+//=> 'Invalid Date'
+is.validDate(invalid);
+//=> false
+```
+
+##### .validLength(value)
+
+Returns `true` if the value is a safe integer that is greater than or equal to zero.
+
+This can be useful to confirm that a value is a valid count of something, ie. 0 or more.
+
+##### .whitespaceString(value)
+
+Returns `true` if the value is a string with only whitespace characters.
+
 ## Type guards
 
 When using `is` together with TypeScript, [type guards](http://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-differentiating-types) are being used extensively to infer the correct type inside if-else statements.
@@ -509,6 +786,61 @@ handleMovieRatingApiResponse({rating: 0.87, title: 'The Matrix'});
 handleMovieRatingApiResponse({rating: '🦄'});
 ```
 
+### Negative assertion
+
+Asserts that `value` is not the specified type. Only exact, type-safe negative assertions are exposed.
+
+Supported assertions:
+
+- `assert.not.undefined(value)`
+- `assert.not.null(value)`
+- `assert.not.nullOrUndefined(value)`
+- `assert.not.string(value)`
+- `assert.not.boolean(value)`
+- `assert.not.symbol(value)`
+- `assert.not.bigint(value)`
+- `assert.not.primitive(value)`
+
+This intentionally excludes checks that cannot produce a safe TypeScript complement: `number` because `is.number` rejects `NaN`, refinements such as `integer` and `validDate`, and branded structural object checks such as `map` and `date`. Broad object checks such as `object` are also excluded to keep negative assertions limited to primitive and nullish types.
+
+```ts
+import {assert} from '@sindresorhus/is';
+
+const value: string | undefined = getValue();
+
+assert.not.undefined(value);
+// Throws if `value` is `undefined`. Otherwise, `value` is now typed as `string`.
+```
+
+For `unknown` input, exact negative assertions narrow to the remaining representable type:
+
+```ts
+const value: unknown = getValue();
+
+assert.not.nullOrUndefined(value);
+// `value` is now typed as non-nullish.
+
+assert.not.primitive(value);
+// `value` is now typed as `object`.
+```
+
+### Optional assertion
+
+Asserts that `value` is `undefined` or satisfies the provided `assertion`.
+
+```ts
+import {assert} from '@sindresorhus/is';
+
+assert.optional(undefined, assert.string);
+// Passes without throwing
+
+assert.optional('🦄', assert.string);
+// Passes without throwing
+
+assert.optional(123, assert.string);
+// Throws: Expected value which is `string`, received value of type `number`
+```
+
 ## Generic type parameters
 
 The type guards and type assertions are aware of [generic type parameters](https://www.typescriptlang.org/docs/handbook/generics.html), such as `Promise<T>` and `Map<Key, Value>`. The default is `unknown` for most cases, since `is` cannot check them at runtime. If the generic type is known at compile-time, either implicitly (inferred) or explicitly (provided), `is` propagates the type so it can be used later.
@@ -568,15 +900,13 @@ For the ones I found, pick 3 of these.
 
 The most common mistakes I noticed in these modules was using `instanceof` for type checking, forgetting that functions are objects, and omitting `symbol` as a primitive.
 
-## For enterprise
+### Why not just use `instanceof` instead of this package?
 
-Available as part of the Tidelift Subscription.
-
-The maintainers of @sindresorhus/is and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. [Learn more.](https://tidelift.com/subscription/pkg/npm-sindresorhus-is?utm_source=npm-sindresorhus-is&utm_medium=referral&utm_campaign=enterprise&utm_term=repo)
+`instanceof` does not work correctly for all types and it does not work across [realms](https://stackoverflow.com/a/49832343/64949). Examples of realms are iframes, windows, web workers, and the `vm` module in Node.js.
 
 ## Related
 
-- [ow](https://github.com/sindresorhus/ow) - Function argument validation for humans
+- [environment](https://github.com/sindresorhus/environment) - Check which JavaScript environment your code is running in at runtime
 - [is-stream](https://github.com/sindresorhus/is-stream) - Check if something is a Node.js stream
 - [is-observable](https://github.com/sindresorhus/is-observable) - Check if a value is an Observable
 - [file-type](https://github.com/sindresorhus/file-type) - Detect the file type of a Buffer/Uint8Array

source/types.ts

@@ -0,0 +1,199 @@
+// Extracted from https://github.com/sindresorhus/type-fest/blob/78019f42ea888b0cdceb41a4a78163868de57555/index.d.ts
+
+/**
+Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive).
+*/
+export type Primitive =
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	| null
+	| undefined
+	| string
+	| number
+	| boolean
+	| symbol
+	| bigint;
+
+/**
+Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
+*/
+type Constructor<T, Arguments extends unknown[] = any[]> = new(...arguments_: Arguments) => T;
+
+/**
+Matches a [`class`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
+*/
+export type Class<T, Arguments extends unknown[] = any[]> = Constructor<T, Arguments> & {prototype: T};
+
+/**
+Matches any [typed array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray), like `Uint8Array` or `Float64Array`.
+*/
+export type TypedArray =
+	| Int8Array
+	| Uint8Array
+	| Uint8ClampedArray
+	| Int16Array
+	| Uint16Array
+	| Int32Array
+	| Uint32Array
+	| Float32Array
+	| Float64Array
+	| BigInt64Array
+	| BigUint64Array;
+
+declare global {
+	// eslint-disable-next-line @typescript-eslint/consistent-type-definitions -- This must be an `interface` so it can be merged.
+	interface SymbolConstructor {
+		readonly observable: symbol;
+	}
+}
+
+/**
+Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable).
+*/
+export type ObservableLike = {
+	subscribe(observer: (value: unknown) => void): void;
+	[Symbol.observable](): ObservableLike;
+};
+
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+export type Falsy = false | 0 | 0n | '' | null | undefined;
+
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+export type WeakRef<T extends object> = {
+	readonly [Symbol.toStringTag]: 'WeakRef';
+	deref(): T | undefined;
+};
+
+export type ArrayLike<T> = {
+	readonly [index: number]: T;
+	readonly length: number;
+};
+
+export type NodeStream = {
+	pipe<T extends NodeJS.WritableStream>(destination: T, options?: {end?: boolean}): T;
+} & NodeJS.EventEmitter;
+
+export type Predicate = (value: unknown) => boolean;
+
+export type NonEmptyString = string & {0: string};
+
+export type Whitespace = ' ';
+
+type Brand<Key extends string> = Readonly<Record<Key, true>>;
+
+/**
+A string that represents a valid URL.
+
+This is a branded type to prevent incorrect TypeScript type narrowing.
+*/
+export type UrlString = string & {readonly __brand: 'UrlString'};
+
+// Keep numeric guards branded and simple. This intentionally favors correct false-branch narrowing for `number` inputs over perfect success-branch narrowing for numeric literal unions.
+
+/**
+The IEEE 754 "Not-a-Number" value, typed as a subtype of `number`.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NaN = number & Brand<'__nanBrand'>;
+
+/**
+A finite number (excludes `NaN`, `Infinity`, and `-Infinity`).
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type FiniteNumber = number & Brand<'__finiteNumberBrand'>;
+
+/**
+A number greater than or equal to zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NonNegativeNumber = number & Brand<'__nonNegativeNumberBrand'>;
+
+/**
+An integer value (no fractional part).
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type Integer = FiniteNumber & Brand<'__integerBrand'>;
+
+/**
+A number greater than zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type PositiveNumber = NonNegativeNumber & Brand<'__positiveNumberBrand'>;
+
+/**
+A number less than zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NegativeNumber = number & Brand<'__negativeNumberBrand'>;
+
+/**
+An integer less than zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NegativeInteger = Integer & NegativeNumber & Brand<'__negativeIntegerBrand'>;
+
+/**
+An integer greater than or equal to zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NonNegativeInteger = Integer & NonNegativeNumber & Brand<'__nonNegativeIntegerBrand'>;
+
+/**
+An integer greater than zero.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type PositiveInteger = NonNegativeInteger & PositiveNumber & Brand<'__positiveIntegerBrand'>;
+
+// Note: type-fest uses the `1e999` overflow trick to represent these types (since TypeScript has
+// no built-in Infinity type), but we use branded types here for consistency and to avoid
+// relying on numeric overflow behavior.
+
+/**
+A positive infinite number (`Infinity`).
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type PositiveInfinity = PositiveNumber & Brand<'__positiveInfinityBrand'>;
+
+/**
+A negative infinite number (`-Infinity`).
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type NegativeInfinity = NegativeNumber & Brand<'__negativeInfinityBrand'>;
+
+/**
+A safe integer (within the range of `Number.MIN_SAFE_INTEGER` to `Number.MAX_SAFE_INTEGER`).
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type SafeInteger = Integer & Brand<'__safeIntegerBrand'>;
+
+/**
+An even integer.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type EvenInteger = Integer & Brand<'__evenIntegerBrand'>;
+
+/**
+An odd integer.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type OddInteger = Integer & Brand<'__oddIntegerBrand'>;
+
+/**
+A non-negative safe integer, suitable as an array or string length.
+
+Branded to prevent false-branch narrowing to `never` when the input is `number`.
+*/
+export type ValidLength = SafeInteger & NonNegativeInteger & Brand<'__validLengthBrand'>;

source/utilities.ts

@@ -0,0 +1,3 @@
+export function keysOf<T extends Record<PropertyKey, unknown>>(value: T): Array<keyof T> {
+	return Object.keys(value) as Array<keyof T>; // eslint-disable-line @typescript-eslint/no-unnecessary-type-assertion
+}

test/tsconfig.json

@@ -0,0 +1,12 @@
+{
+	"extends": "../tsconfig.json",
+	"compilerOptions": {
+		"rootDir": "..",
+		"noUnusedLocals": false,
+		"noUnusedParameters": false
+	},
+	"include": [
+		"../source",
+		"type-tests.ts"
+	]
+}

test/type-tests.ts

@@ -0,0 +1,405 @@
+import {expectTypeOf} from 'expect-type';
+import is, {
+	assert as isAssert,
+	assertNotNullOrUndefined,
+	assertNotPrimitive,
+	assertNotString,
+	assertNotUndefined,
+	type EvenInteger,
+	type FiniteNumber,
+	type Integer,
+	type NaN as NaNType,
+	type NegativeInfinity,
+	type NegativeInteger,
+	type NegativeNumber,
+	type NonNegativeInteger,
+	type NonNegativeNumber,
+	type OddInteger,
+	type PositiveInfinity,
+	type PositiveInteger,
+	type PositiveNumber,
+	type Primitive,
+	type SafeInteger,
+	type ValidLength,
+} from '../source/index.ts';
+
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+type UnknownNotPrimitive<Forbidden extends Primitive> = Exclude<Primitive, Forbidden> | object;
+
+// For each predicate, verify two things:
+// 1. True branch narrows to the branded type.
+// 2. False branch on a `number` input stays `number` (not `never`).
+//    Without the branded types, `Exclude<number, number>` = `never` would break
+//    the common validation-guard pattern: if (!is.X(n)) throw; use(n).
+
+const nanCheck = (value: number) => {
+	if (is.nan(value)) {
+		const _: NaNType = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const finiteNumberCheck = (value: number) => {
+	if (is.finiteNumber(value)) {
+		const _: FiniteNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const nonNegativeNumberCheck = (value: number) => {
+	if (is.nonNegativeNumber(value)) {
+		const _: NonNegativeNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const positiveIntegerCheck = (value: number) => {
+	if (is.positiveInteger(value)) {
+		const _: PositiveInteger = value;
+		const __: Integer = value;
+		const ___: NonNegativeInteger = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const negativeIntegerCheck = (value: number) => {
+	if (is.negativeInteger(value)) {
+		const _: NegativeInteger = value;
+		const __: Integer = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const nonNegativeIntegerCheck = (value: number) => {
+	if (is.nonNegativeInteger(value)) {
+		const _: NonNegativeInteger = value;
+		const __: Integer = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const infiniteCheck = (value: number) => {
+	if (is.infinite(value)) {
+		const _: PositiveInfinity | NegativeInfinity = value;
+		const __: PositiveNumber | NegativeNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const integerCheck = (value: number) => {
+	if (is.integer(value)) {
+		const _: Integer = value;
+		const __: FiniteNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const safeIntegerCheck = (value: number) => {
+	if (is.safeInteger(value)) {
+		const _: SafeInteger = value;
+		const __: Integer = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const evenIntegerCheck = (value: number) => {
+	if (is.evenInteger(value)) {
+		const _: EvenInteger = value;
+		const __: Integer = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const oddIntegerCheck = (value: number) => {
+	if (is.oddInteger(value)) {
+		const _: OddInteger = value;
+		const __: Integer = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const positiveNumberCheck = (value: number) => {
+	if (is.positiveNumber(value)) {
+		const _: PositiveNumber = value;
+		const __: NonNegativeNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const negativeNumberCheck = (value: number) => {
+	if (is.negativeNumber(value)) {
+		const _: NegativeNumber = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const validLengthCheck = (value: number) => {
+	if (is.validLength(value)) {
+		const _: ValidLength = value;
+		const __: SafeInteger = value;
+		const ___: NonNegativeInteger = value;
+	} else {
+		const _: number = value;
+	}
+};
+
+const integerUnknownCheck = (value: unknown) => {
+	if (is.integer(value)) {
+		const _: Integer = value;
+		const __: FiniteNumber = value;
+	}
+};
+
+const positiveIntegerUnknownCheck = (value: unknown) => {
+	if (is.positiveInteger(value)) {
+		const _: PositiveInteger = value;
+		const __: NonNegativeInteger = value;
+	}
+};
+
+const integerMixedUnionCheck = (value: string | number) => {
+	if (is.integer(value)) {
+		const _: number = value;
+	} else {
+		const _: string = value;
+	}
+};
+
+const positiveNumberMixedUnionCheck = (value: string | number) => {
+	if (is.positiveNumber(value)) {
+		const _: number = value;
+	} else {
+		const _: string = value;
+	}
+};
+
+const chainedNumericGuardCheck = (value: number) => {
+	if (is.positiveNumber(value) && is.integer(value)) {
+		const _: PositiveNumber = value;
+		const __: Integer = value;
+		const ___: FiniteNumber = value;
+	}
+};
+
+const distinctNumericBrandsStayDistinct = (
+	positiveInteger: PositiveInteger,
+	negativeInteger: NegativeInteger,
+	validLength: ValidLength,
+) => {
+	// @ts-expect-error -- Distinct numeric refinements must not collapse into each other.
+	const _: NegativeInteger = positiveInteger;
+	// @ts-expect-error -- ValidLength is non-negative and must not become a signed integer refinement.
+	const __: NegativeInteger = validLength;
+
+	return negativeInteger;
+};
+
+const assertNotUndefinedCheck = (value: string | undefined) => {
+	isAssert.not.undefined(value);
+	expectTypeOf(value).toEqualTypeOf<string>();
+};
+
+const assertNotUndefinedUnknownCheck = (value: unknown) => {
+	isAssert.not.undefined(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<undefined>>();
+};
+
+const assertNotUndefinedGenericCheck = <T>(value: T) => {
+	isAssert.not.undefined(value);
+	const _: Exclude<T, undefined> = value;
+};
+
+const nullValue = null;
+type Null = typeof nullValue;
+
+const assertNotNullUnknownCheck = (value: unknown) => {
+	isAssert.not.null(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<Null>>();
+};
+
+const assertNotNullOrUndefinedCheck = (value: string | Null | undefined) => {
+	isAssert.not.nullOrUndefined(value);
+	expectTypeOf(value).toEqualTypeOf<string>();
+};
+
+const assertNotNullOrUndefinedUnknownCheck = (value: unknown) => {
+	isAssert.not.nullOrUndefined(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<Null | undefined>>();
+};
+
+const assertNotStringCheck = (value: string | number) => {
+	isAssert.not.string(value);
+	expectTypeOf(value).toEqualTypeOf<number>();
+};
+
+const assertNotStringUnknownCheck = (value: unknown) => {
+	isAssert.not.string(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<string>>();
+};
+
+const assertNotStringGenericCheck = <T>(value: T) => {
+	isAssert.not.string(value);
+	const _: Exclude<T, string> = value;
+};
+
+const assertNotBooleanUnknownCheck = (value: unknown) => {
+	isAssert.not.boolean(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<boolean>>();
+};
+
+const assertNotSymbolUnknownCheck = (value: unknown) => {
+	isAssert.not.symbol(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<symbol>>();
+};
+
+const assertNotBigintUnknownCheck = (value: unknown) => {
+	isAssert.not.bigint(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<bigint>>();
+};
+
+const assertNotPrimitiveUnknownCheck = (value: unknown) => {
+	isAssert.not.primitive(value);
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	expectTypeOf(value).toEqualTypeOf<object>();
+};
+
+const assertNotPrimitiveGenericCheck = <T>(value: T) => {
+	isAssert.not.primitive(value);
+	const _: Exclude<T, Primitive> = value;
+};
+
+const assertNotNamedUndefinedExportCheck = (value: 0 | false | '' | Null | undefined | 'ok') => {
+	assertNotUndefined(value);
+	expectTypeOf(value).toEqualTypeOf<0 | false | '' | Null | 'ok'>();
+};
+
+const assertNotNamedNullOrUndefinedUnknownExportCheck = (value: unknown) => {
+	assertNotNullOrUndefined(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<Null | undefined>>();
+};
+
+const assertNotNamedStringExportCheck = (value: string | number) => {
+	assertNotString(value);
+	expectTypeOf(value).toEqualTypeOf<number>();
+};
+
+const assertNotNamedStringUnknownExportCheck = (value: unknown) => {
+	assertNotString(value);
+	expectTypeOf(value).toEqualTypeOf<UnknownNotPrimitive<string>>();
+};
+
+const assertNotNamedPrimitiveUnknownExportCheck = (value: unknown) => {
+	assertNotPrimitive(value);
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	expectTypeOf(value).toEqualTypeOf<object>();
+};
+
+const assertNotCallableDoesNotExistCheck = (value: string | undefined) => {
+	// @ts-expect-error -- Generic negative assertions cannot safely infer complement types from arbitrary predicates.
+	isAssert.not(is.undefined, value);
+	const _: string | undefined = value;
+};
+
+const assertNotNumberDoesNotExistCheck = (value: string | number) => {
+	// @ts-expect-error -- `is.number` rejects `NaN`, so a narrowing negative assertion would be unsound.
+	isAssert.not.number(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: string | number = value;
+};
+
+const assertNotIntegerDoesNotExistCheck = (value: string | number) => {
+	// @ts-expect-error -- Numeric refinements are intentionally excluded from `assert.not`.
+	isAssert.not.integer(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: string | number = value;
+};
+
+const assertNotObjectDoesNotExistCheck = (value: Record<string, unknown> | string) => {
+	// @ts-expect-error -- TypeScript's `{}` type includes primitives, so `not.object` cannot safely narrow every object-like input.
+	isAssert.not.object(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: Record<string, unknown> | string = value;
+};
+
+const assertNotBlobDoesNotExistCheck = (value: Blob | File | string) => {
+	// @ts-expect-error -- `File` extends `Blob` in TypeScript but does not match the exact runtime `Blob` check.
+	isAssert.not.blob(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: Blob | File | string = value;
+};
+
+const assertNotMapDoesNotExistCheck = (value: Map<string, number> | string) => {
+	// @ts-expect-error -- Structural object types such as `Map` can be assignable in TypeScript without matching the runtime brand check.
+	isAssert.not.map(value); // eslint-disable-line @typescript-eslint/no-unsafe-call, unicorn/no-array-callback-reference
+	const _: Map<string, number> | string = value;
+};
+
+const assertNotSetDoesNotExistCheck = (value: Set<string> | string) => {
+	// @ts-expect-error -- Structural object types such as `Set` can be assignable in TypeScript without matching the runtime brand check.
+	isAssert.not.set(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: Set<string> | string = value;
+};
+
+const assertNotDateDoesNotExistCheck = (value: Date | string) => {
+	// @ts-expect-error -- Structural object types such as `Date` can be assignable in TypeScript without matching the runtime brand check.
+	isAssert.not.date(value); // eslint-disable-line @typescript-eslint/no-unsafe-call
+	const _: Date | string = value;
+};
+
+// Suppress unused variable warnings
+nanCheck(42);
+finiteNumberCheck(42);
+nonNegativeNumberCheck(42);
+positiveIntegerCheck(42);
+negativeIntegerCheck(-1);
+nonNegativeIntegerCheck(0);
+infiniteCheck(Number.POSITIVE_INFINITY);
+integerCheck(1);
+safeIntegerCheck(1);
+evenIntegerCheck(2);
+oddIntegerCheck(1);
+positiveNumberCheck(1);
+negativeNumberCheck(-1);
+validLengthCheck(0);
+integerUnknownCheck(1);
+positiveIntegerUnknownCheck(1);
+integerMixedUnionCheck(1);
+positiveNumberMixedUnionCheck(1);
+chainedNumericGuardCheck(1);
+distinctNumericBrandsStayDistinct(42 as PositiveInteger, -1 as NegativeInteger, 0 as ValidLength);
+assertNotUndefinedCheck('🦄');
+assertNotUndefinedUnknownCheck('🦄');
+assertNotUndefinedGenericCheck<string | undefined>('🦄');
+assertNotNullUnknownCheck('🦄');
+assertNotNullOrUndefinedCheck('🦄');
+assertNotNullOrUndefinedUnknownCheck('🦄');
+assertNotStringCheck(1);
+assertNotStringUnknownCheck(1);
+assertNotStringGenericCheck<string | number>(1);
+assertNotBooleanUnknownCheck(1);
+assertNotSymbolUnknownCheck(1);
+assertNotBigintUnknownCheck(1);
+assertNotPrimitiveUnknownCheck({});
+assertNotPrimitiveGenericCheck<string | {unicorn: true}>({unicorn: true});
+assertNotNamedUndefinedExportCheck(0);
+assertNotNamedNullOrUndefinedUnknownExportCheck('🦄');
+assertNotNamedStringExportCheck(1);
+assertNotNamedStringUnknownExportCheck(1);
+assertNotNamedPrimitiveUnknownExportCheck({});
+assertNotCallableDoesNotExistCheck('🦄');
+assertNotNumberDoesNotExistCheck(Number.NaN);
+assertNotIntegerDoesNotExistCheck(1.5);
+assertNotObjectDoesNotExistCheck('🦄');
+assertNotBlobDoesNotExistCheck('🦄');
+assertNotMapDoesNotExistCheck('🦄');
+assertNotSetDoesNotExistCheck('🦄');
+assertNotDateDoesNotExistCheck('🦄');

tsconfig.json

@@ -1,14 +1,12 @@
 {
 	"extends": "@sindresorhus/tsconfig",
 	"compilerOptions": {
-		"outDir": "dist",
-		"target": "es2018",
-		"lib": [
-			"es2018",
-			"dom"
-		]
+		"types": ["node"],
+		"rootDir": "source",
+		"allowImportingTsExtensions": true,
+		"rewriteRelativeImportExtensions": true
 	},
 	"include": [
 		"source"
-	]
+	],
 }

tsconfig.xo.json

@@ -1,6 +0,0 @@
-{
-	"extends": "./tsconfig.json",
-	"include": [
-		"test"
-	]
-}