Fix some type guards

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

@@ -21,7 +21,7 @@
 	},
 	"scripts": {
 		"build": "del distribution && tsc",
-		"test": "tsc --noEmit && xo && node --experimental-transform-types --test test/test.ts",
+		"test": "tsc --noEmit && tsc --project test/tsconfig.json --noEmit && xo && node --experimental-transform-types --test test/test.ts",
 		"prepare": "npm run build"
 	},
 	"files": [

source/index.ts

@@ -1,14 +1,29 @@
 import type {
 	ArrayLike,
 	Class,
+	EvenInteger,
 	Falsy,
+	FiniteNumber,
+	Integer,
+	NaN as NaNType,
+	NegativeInfinity,
+	NegativeInteger,
+	NegativeNumber,
 	NodeStream,
 	NonEmptyString,
+	NonNegativeInteger,
+	NonNegativeNumber,
 	ObservableLike,
+	OddInteger,
 	Predicate,
 	Primitive,
+	PositiveInfinity,
+	PositiveInteger,
+	PositiveNumber,
+	SafeInteger,
 	TypedArray,
 	UrlString,
+	ValidLength,
 	WeakRef,
 	Whitespace,
 } from './types.ts';
@@ -22,6 +37,15 @@ type ExtractFromGlobalConstructors<Name extends string> =
 
 type NodeBuffer = ExtractFromGlobalConstructors<'Buffer'>;
 
+type NumericGuardResult<Input, Branded extends number> =
+	(
+		unknown extends Input
+			? Branded
+			: Input extends number
+				? Branded & Input
+				: number
+	) & Input;
+
 const typedArrayTypeNames = [
 	'Int8Array',
 	'Uint8Array',
@@ -99,6 +123,7 @@ function isPrimitiveTypeName(name: unknown): name is PrimitiveTypeName {
 export type TypeName = ObjectTypeName | PrimitiveTypeName;
 
 const assertionTypeDescriptions = [
+	'bound Function',
 	'positive number',
 	'negative number',
 	'Class',
@@ -139,6 +164,7 @@ const assertionTypeDescriptions = [
 	'non-negative number',
 	'odd integer',
 	'positive integer',
+	'safe integer',
 	'T',
 	'in range',
 	'predicate returns truthy for any value',
@@ -225,8 +251,7 @@ function detect(value: unknown): TypeName {
 		return 'Promise';
 	}
 
-	const objectTag = Object.prototype.toString.call(value).slice(8, -1);
+	if (isBoxedPrimitiveObject(value)) {
-	if (objectTag === 'String' || objectTag === 'Boolean' || objectTag === 'Number') {
 		throw new TypeError('Please don\'t use object wrappers for primitive types');
 	}
 
@@ -237,6 +262,23 @@ function hasPromiseApi<T = unknown>(value: unknown): value is Promise<T> {
 	return isFunction((value as Promise<T>)?.then) && isFunction((value as Promise<T>)?.catch);
 }
 
+function hasBoxedPrimitiveBrand(value: unknown, valueOf: () => unknown): boolean {
+	try {
+		// `Object.prototype.toString` can be spoofed via `Symbol.toStringTag`, but the
+		// boxed primitive `valueOf` methods still enforce the real internal brand.
+		Reflect.apply(valueOf, value, []);
+		return true;
+	} catch {
+		return false;
+	}
+}
+
+function isBoxedPrimitiveObject(value: unknown): boolean {
+	return hasBoxedPrimitiveBrand(value, String.prototype.valueOf)
+		|| hasBoxedPrimitiveBrand(value, Boolean.prototype.valueOf)
+		|| hasBoxedPrimitiveBrand(value, Number.prototype.valueOf);
+}
+
 const is = Object.assign(
 	detect,
 	{
@@ -560,11 +602,13 @@ export function isEnumCase<T = unknown>(value: unknown, targetEnum: T): value is
 }
 
 export function isError(value: unknown): value is Error {
-	// TODO: Use `Error.isError` when targeting Node.js 24.`
+	// TODO: Use `Error.isError` when targeting Node.js 24.
 	return getObjectType(value) === 'Error';
 }
 
-export function isEvenInteger(value: unknown): value is number {
+// For numeric guards, preserve branded narrowing for `unknown`, keep the false branch usable for plain `number`, and still narrow mixed unions to `number`.
+export function isEvenInteger<Input>(value: Input): value is NumericGuardResult<Input, EvenInteger>;
+export function isEvenInteger(value: unknown): boolean {
 	return isAbsoluteModule2(0)(value);
 }
 
@@ -573,7 +617,8 @@ export function isFalsy(value: unknown): value is Falsy {
 	return !value;
 }
 
-export function isFiniteNumber(value: unknown): value is number {
+export function isFiniteNumber<Input>(value: Input): value is NumericGuardResult<Input, FiniteNumber>;
+export function isFiniteNumber(value: unknown): boolean {
 	return Number.isFinite(value);
 }
 
@@ -622,7 +667,8 @@ export function isHtmlElement(value: unknown): value is HTMLElement {
 		&& DOM_PROPERTIES_TO_CHECK.every(property => property in value);
 }
 
-export function isInfinite(value: unknown): value is number {
+export function isInfinite<Input>(value: Input): value is NumericGuardResult<Input, PositiveInfinity | NegativeInfinity>;
+export function isInfinite(value: unknown): boolean {
 	return value === Number.POSITIVE_INFINITY || value === Number.NEGATIVE_INFINITY;
 }
 
@@ -654,7 +700,8 @@ export function isInt8Array(value: unknown): value is Int8Array {
 	return getObjectType(value) === 'Int8Array';
 }
 
-export function isInteger(value: unknown): value is number {
+export function isInteger<Input>(value: Input): value is NumericGuardResult<Input, Integer>;
+export function isInteger(value: unknown): boolean {
 	return Number.isInteger(value);
 }
 
@@ -666,7 +713,8 @@ export function isMap<Key = unknown, Value = unknown>(value: unknown): value is
 	return getObjectType(value) === 'Map';
 }
 
-export function isNan(value: unknown) {
+export function isNan<Input>(value: Input): value is NumericGuardResult<Input, NaNType>;
+export function isNan(value: unknown): boolean {
 	return Number.isNaN(value);
 }
 
@@ -674,11 +722,13 @@ export function isNativePromise<T = unknown>(value: unknown): value is Promise<T
 	return getObjectType(value) === 'Promise';
 }
 
-export function isNegativeInteger(value: unknown): value is number {
+export function isNegativeInteger<Input>(value: Input): value is NumericGuardResult<Input, NegativeInteger>;
+export function isNegativeInteger(value: unknown): boolean {
 	return isInteger(value) && value < 0;
 }
 
-export function isNegativeNumber(value: unknown): value is number {
+export function isNegativeNumber<Input>(value: Input): value is NumericGuardResult<Input, NegativeNumber>;
+export function isNegativeNumber(value: unknown): boolean {
 	return isNumber(value) && value < 0;
 }
 
@@ -714,11 +764,13 @@ export function isNonEmptyStringAndNotWhitespace(value: unknown): value is NonEm
 	return isString(value) && !isEmptyStringOrWhitespace(value);
 }
 
-export function isNonNegativeInteger(value: unknown): value is number {
+export function isNonNegativeInteger<Input>(value: Input): value is NumericGuardResult<Input, NonNegativeInteger>;
+export function isNonNegativeInteger(value: unknown): boolean {
 	return isInteger(value) && value >= 0;
 }
 
-export function isNonNegativeNumber(value: unknown): value is number {
+export function isNonNegativeNumber<Input>(value: Input): value is NumericGuardResult<Input, NonNegativeNumber>;
+export function isNonNegativeNumber(value: unknown): boolean {
 	return isNumber(value) && value >= 0;
 }
 
@@ -763,7 +815,8 @@ export function isObservable(value: unknown): value is ObservableLike {
 	return false;
 }
 
-export function isOddInteger(value: unknown): value is number {
+export function isOddInteger<Input>(value: Input): value is NumericGuardResult<Input, OddInteger>;
+export function isOddInteger(value: unknown): boolean {
 	return isAbsoluteModule2(1)(value);
 }
 
@@ -783,11 +836,13 @@ export function isPlainObject<Value = unknown>(value: unknown): value is Record<
 	return (prototype === null || prototype === Object.prototype || Object.getPrototypeOf(prototype) === null) && !(Symbol.toStringTag in value) && !(Symbol.iterator in value);
 }
 
-export function isPositiveInteger(value: unknown): value is number {
+export function isPositiveInteger<Input>(value: Input): value is NumericGuardResult<Input, PositiveInteger>;
+export function isPositiveInteger(value: unknown): boolean {
 	return isInteger(value) && value > 0;
 }
 
-export function isPositiveNumber(value: unknown): value is number {
+export function isPositiveNumber<Input>(value: Input): value is NumericGuardResult<Input, PositiveNumber>;
+export function isPositiveNumber(value: unknown): boolean {
 	return isNumber(value) && value > 0;
 }
 
@@ -808,7 +863,8 @@ export function isRegExp(value: unknown): value is RegExp {
 	return getObjectType(value) === 'RegExp';
 }
 
-export function isSafeInteger(value: unknown): value is number {
+export function isSafeInteger<Input>(value: Input): value is NumericGuardResult<Input, SafeInteger>;
+export function isSafeInteger(value: unknown): boolean {
 	return Number.isSafeInteger(value);
 }
 
@@ -900,7 +956,8 @@ export function isValidDate(value: unknown): value is Date {
 	return isDate(value) && !isNan(Number(value));
 }
 
-export function isValidLength(value: unknown): value is number {
+export function isValidLength<Input>(value: Input): value is NumericGuardResult<Input, ValidLength>;
+export function isValidLength(value: unknown): boolean {
 	return isSafeInteger(value) && value >= 0;
 }
 
@@ -956,6 +1013,8 @@ function typeErrorMessageMultipleValues(expectedType: AssertionTypeDescription |
 }
 
 // Type assertions have to be declared with an explicit type.
+// Keep assertion outputs unbranded even when the corresponding `is.*` guard uses a branded subtype.
+// The brands exist to preserve useful false-branch narrowing for type guards on `number` inputs, which does not apply to `asserts`.
 type Assert = {
 	// Unknowns.
 	undefined: (value: unknown, message?: string) => asserts value is undefined;
@@ -1188,7 +1247,7 @@ const methodTypeMap = {
 	isBigUint64Array: 'BigUint64Array',
 	isBlob: 'Blob',
 	isBoolean: 'boolean',
-	isBoundFunction: 'Function',
+	isBoundFunction: 'bound Function',
 	isBuffer: 'Buffer',
 	isClass: 'Class',
 	isDataView: 'DataView',
@@ -1247,7 +1306,7 @@ const methodTypeMap = {
 	isPromise: 'Promise',
 	isPropertyKey: 'PropertyKey',
 	isRegExp: 'RegExp',
-	isSafeInteger: 'integer',
+	isSafeInteger: 'safe integer',
 	isSet: 'Set',
 	isSharedArrayBuffer: 'SharedArrayBuffer',
 	isString: 'string',
@@ -1391,7 +1450,7 @@ export function assertBoolean(value: unknown, message?: string): asserts value i
 // eslint-disable-next-line @typescript-eslint/no-unsafe-function-type
 export function assertBoundFunction(value: unknown, message?: string): asserts value is Function {
 	if (!isBoundFunction(value)) {
-		throw new TypeError(message ?? typeErrorMessage('Function', value));
+		throw new TypeError(message ?? typeErrorMessage('bound Function', value));
 	}
 }
 
@@ -1752,7 +1811,7 @@ export function assertRegExp(value: unknown, message?: string): asserts value is
 
 export function assertSafeInteger(value: unknown, message?: string): asserts value is number {
 	if (!isSafeInteger(value)) {
-		throw new TypeError(message ?? typeErrorMessage('integer', value));
+		throw new TypeError(message ?? typeErrorMessage('safe integer', value));
 	}
 }
 
@@ -1891,10 +1950,25 @@ export default is;
 export type {
 	ArrayLike,
 	Class,
+	EvenInteger,
+	FiniteNumber,
+	Integer,
+	NaN,
+	NegativeInfinity,
+	NegativeInteger,
+	NegativeNumber,
 	NodeStream,
+	NonNegativeInteger,
+	NonNegativeNumber,
 	ObservableLike,
+	OddInteger,
+	PositiveInfinity,
+	PositiveInteger,
+	PositiveNumber,
 	Predicate,
 	Primitive,
+	SafeInteger,
 	TypedArray,
 	UrlString,
+	ValidLength,
 } from './types.ts';

source/types.ts

@@ -78,9 +78,122 @@ 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'>;

test/test.ts

@@ -15,6 +15,7 @@ import is, {
 	assert as isAssert,
 	assertPropertyKey,
 	type AssertionTypeDescription,
+	type NaN as NaNType,
 	type Predicate,
 	type Primitive,
 	type TypedArray,
@@ -156,7 +157,7 @@ const primitiveTypes = {
 	safeInteger: {
 		fixtures: [...reusableFixtures.integer, ...reusableFixtures.safeInteger],
 		typename: 'number',
-		typeDescription: 'integer',
+		typeDescription: 'safe integer',
 	},
 	infinite: {
 		fixtures: [...reusableFixtures.infinite],
@@ -208,6 +209,7 @@ const objectTypes = {
 	object: {
 		fixtures: [
 			Object.create({x: 1}),
+			{[Symbol.toStringTag]: 'String'},
 			...reusableFixtures.plainObject,
 		],
 		typename: 'Object',
@@ -280,6 +282,7 @@ const objectTypes = {
 	boundFunction: {
 		fixtures: [...reusableFixtures.boundFunction, ...reusableFixtures.asyncFunction],
 		typename: 'Function',
+		typeDescription: 'bound Function',
 	},
 	map: {
 		fixtures: [
@@ -526,6 +529,7 @@ test('is.positiveNumber', () => {
 	assert.strictEqual(is.positiveNumber(-6), false);
 	assert.strictEqual(is.positiveNumber(-1.4), false);
 	assert.strictEqual(is.positiveNumber(Number.NEGATIVE_INFINITY), false);
+	assert.strictEqual(is.positiveNumber(Number.NaN), false);
 
 	assert.throws(() => {
 		isAssert.positiveNumber(0);
@@ -544,6 +548,33 @@ test('is.positiveNumber', () => {
 	});
 });
 
+test('is.nan', () => {
+	assert.ok(is.nan(Number.NaN));
+	assert.ok(is.nan(NaN)); // eslint-disable-line unicorn/prefer-number-properties
+
+	assert.doesNotThrow(() => {
+		isAssert.nan(Number.NaN);
+	});
+
+	assert.strictEqual(is.nan(0), false);
+	assert.strictEqual(is.nan(-0), false);
+	assert.strictEqual(is.nan(1), false);
+	assert.strictEqual(is.nan(Number.POSITIVE_INFINITY), false);
+	assert.strictEqual(is.nan(Number.NEGATIVE_INFINITY), false);
+	assert.strictEqual(is.nan('NaN'), false);
+	assert.strictEqual(is.nan(undefined), false);
+
+	assert.throws(() => {
+		isAssert.nan(0);
+	});
+	assert.throws(() => {
+		isAssert.nan(1);
+	});
+	assert.throws(() => {
+		isAssert.nan('NaN');
+	});
+});
+
 test('is.finiteNumber', () => {
 	assert.ok(is.finiteNumber(6));
 	assert.ok(is.finiteNumber(-6));
@@ -592,6 +623,7 @@ test('is.negativeNumber', () => {
 	assert.strictEqual(is.negativeNumber(6), false);
 	assert.strictEqual(is.negativeNumber(1.4), false);
 	assert.strictEqual(is.negativeNumber(Number.POSITIVE_INFINITY), false);
+	assert.strictEqual(is.negativeNumber(Number.NaN), false);
 
 	assert.throws(() => {
 		isAssert.negativeNumber(0);
@@ -729,6 +761,32 @@ test('is.nonNegativeInteger', () => {
 	});
 });
 
+test('is.infinite', () => {
+	assert.ok(is.infinite(Number.POSITIVE_INFINITY));
+	assert.ok(is.infinite(Number.NEGATIVE_INFINITY));
+
+	assert.doesNotThrow(() => {
+		isAssert.infinite(Number.POSITIVE_INFINITY);
+	});
+	assert.doesNotThrow(() => {
+		isAssert.infinite(Number.NEGATIVE_INFINITY);
+	});
+
+	assert.strictEqual(is.infinite(0), false);
+	assert.strictEqual(is.infinite(1), false);
+	assert.strictEqual(is.infinite(-1), false);
+	assert.strictEqual(is.infinite(Number.NaN), false);
+	assert.strictEqual(is.infinite(Number.MAX_VALUE), false);
+	assert.strictEqual(is.infinite('Infinity'), false);
+
+	assert.throws(() => {
+		isAssert.infinite(0);
+	});
+	assert.throws(() => {
+		isAssert.infinite(Number.NaN);
+	});
+});
+
 test('is.numericString supplemental', () => {
 	assert.strictEqual(is.numericString(''), false);
 	assert.strictEqual(is.numericString(' '), false);
@@ -998,6 +1056,20 @@ test('is.urlString', () => {
 	}
 })();
 
+// Type test for is.nan branded-type narrowing
+(() => {
+	const value: unknown = Number.NaN;
+
+	if (is.nan(value)) {
+		// ✅ In true branch: value is narrowed to the branded NaN type
+		expectTypeOf(value).toEqualTypeOf<NaNType>();
+		expectTypeOf(value).toMatchTypeOf<number>();
+	} else {
+		// ✅ In false branch: value remains unknown (not incorrectly narrowed)
+		expectTypeOf(value).toEqualTypeOf<unknown>();
+	}
+})();
+
 test('is.truthy', () => {
 	assert.ok(is.truthy('unicorn'));
 	assert.ok(is.truthy('🦄'));
@@ -2268,370 +2340,379 @@ test('assert', () => {
 test('custom assertion message', () => {
 	const message = 'Custom error message';
 
-	assert.throws(() => {
+	const assertThrowsTypeErrorWithMessage = (assertion: () => void) => {
+		// `node:assert` does not verify the error class when matching only on `{message}`.
+		assert.throws(assertion, error => {
+			assert.ok(error instanceof TypeError);
+			assert.strictEqual(error.message, message);
+			return true;
+		});
+	};
+
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.array(undefined, undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.arrayBuffer(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.arrayLike(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.asyncFunction(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.asyncGenerator(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.asyncGeneratorFunction(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.asyncIterable(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.bigInt64Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.bigUint64Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.bigint(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.blob(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.boolean(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.boundFunction(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.buffer(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.class(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.dataView(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.date(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.directInstanceOf(undefined, Error, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptyArray(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptyMap(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptyObject(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptySet(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptyString(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.emptyStringOrWhitespace(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		enum Enum {}
 		isAssert.enumCase('invalid', Enum, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.error(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.evenInteger(33, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.falsy(true, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.finiteNumber(Number.POSITIVE_INFINITY, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.float32Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.float64Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.formData(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.function(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.generator(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.generatorFunction(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.htmlElement(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.inRange(5, [1, 2], message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.infinite(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.int16Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.int32Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.int8Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.integer(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.iterable(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.map(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nan(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nativePromise(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.negativeNumber(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nodeStream(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptyArray(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptyMap(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptyObject(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptySet(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptyString(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonEmptyStringAndNotWhitespace(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nonNegativeNumber(-1, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.null(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.nullOrUndefined(false, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.number(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.numericString(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.object(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.observable(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.oddInteger(42, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.plainObject(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.positiveInteger(0, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.positiveNumber(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.primitive([], message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.promise(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.propertyKey(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.regExp(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.safeInteger(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.set(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.sharedArrayBuffer(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.string(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.symbol(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.truthy(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.tupleLike(undefined, [], message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.typedArray(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.uint16Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.uint32Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.uint8Array(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.uint8ClampedArray(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.undefined(false, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.urlInstance(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.urlSearchParams(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.urlString(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.validDate(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.validLength(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.weakMap(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.weakRef(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.weakSet(undefined, message);
-	}, {message});
+	});
 
-	assert.throws(() => {
+	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.whitespaceString(undefined, message);
-	}, {message});
+	});
 });
 
 test('is.optional', () => {

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 +1,220 @@
+import is, {
+	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 SafeInteger,
+	type ValidLength,
+} from '../source/index.ts';
+
+// 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;
+};
+
+// 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);