Add negative assertion helper

Fixes #220

readme.md

@@ -786,6 +786,44 @@ 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`.

source/index.ts

@@ -403,10 +403,14 @@ function validatePredicateArray(predicateArray: readonly Predicate[], allowEmpty
 	}
 
 	for (const predicate of predicateArray) {
+		validatePredicate(predicate);
+	}
+}
+
+function validatePredicate(predicate: Predicate) {
 	if (!isFunction(predicate)) {
 		throw new TypeError(`Invalid predicate: ${JSON.stringify(predicate)}`);
 	}
-	}
 }
 
 // Predicate factory overloads - return a type guard when called with only predicates
@@ -983,9 +987,7 @@ export function isWhitespaceString(value: unknown): value is Whitespace {
 type ArrayMethod = (function_: (value: unknown, index: number, array: unknown[]) => boolean, thisArgument?: unknown) => boolean;
 
 function predicateOnArray(method: ArrayMethod, predicate: Predicate, values: unknown[]) {
-	if (!isFunction(predicate)) {
-		throw new TypeError(`Invalid predicate: ${JSON.stringify(predicate)}`);
-	}
+	validatePredicate(predicate);
 
 	if (values.length === 0) {
 		throw new TypeError('Invalid number of values');
@@ -998,6 +1000,17 @@ function typeErrorMessage(description: AssertionTypeDescription, value: unknown)
 	return `Expected value which is \`${description}\`, received value of type \`${is(value)}\`.`;
 }
 
+function typeErrorMessageNot(description: AssertionTypeDescription, value: unknown): string {
+	return `Expected value which is not \`${description}\`, received value of type \`${is(value)}\`.`;
+}
+
+type NotAssertionResult<Value, Forbidden, UnknownResult> = Exclude<Value, Forbidden> & ([unknown] extends [Value] ? UnknownResult : unknown);
+
+type NotAssertion<Forbidden, UnknownResult = unknown> = <Value>(value: Value, message?: string) => asserts value is NotAssertionResult<Value, Forbidden, UnknownResult>;
+
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+type UnknownNotPrimitive<Forbidden extends Primitive> = Exclude<Primitive, Forbidden> | object;
+
 function unique<T>(values: T[]): T[] {
 	// eslint-disable-next-line unicorn/prefer-spread
 	return Array.from(new Set(values));
@@ -1123,6 +1136,8 @@ type Assert = {
 	directInstanceOf: <T>(instance: unknown, class_: Class<T>, message?: string) => asserts instance is T;
 	inRange: (value: number, range: number | [number, number], message?: string) => asserts value is number;
 
+	not: NotAssert;
+
 	// Variadic functions.
 	any: (predicate: Predicate | readonly Predicate[], ...values: unknown[]) => void | never;
 	all: (predicate: Predicate | readonly Predicate[], ...values: unknown[]) => void | never;
@@ -1135,9 +1150,58 @@ type Assert = {
 	optional: <T>(value: unknown, assertion: (value: unknown, message?: string) => asserts value is T, message?: string) => asserts value is T | undefined;
 };
 
+type NotAssert = {
+	undefined: NotAssertion<undefined, UnknownNotPrimitive<undefined>>;
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	null: NotAssertion<null, UnknownNotPrimitive<null>>;
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	nullOrUndefined: NotAssertion<null | undefined, UnknownNotPrimitive<null | undefined>>;
+	string: NotAssertion<string, UnknownNotPrimitive<string>>;
+	boolean: NotAssertion<boolean, UnknownNotPrimitive<boolean>>;
+	symbol: NotAssertion<symbol, UnknownNotPrimitive<symbol>>;
+	bigint: NotAssertion<bigint, UnknownNotPrimitive<bigint>>;
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	primitive: NotAssertion<Primitive, object>;
+};
+
+// Negative assertions are limited to types where the assertion rejects every TypeScript value assignable to the forbidden type. Structural object types such as `Map`, `Set`, `Date`, and `Array` are excluded because TypeScript accepts shape-compatible mocks while the runtime checks use object brands, so `Exclude` would narrow values that can pass the negative assertion.
+function createAssertNot<Forbidden, UnknownResult = unknown>(predicate: Predicate, description: AssertionTypeDescription): NotAssertion<Forbidden, UnknownResult> {
+	return <Value>(value: Value, message?: string): asserts value is NotAssertionResult<Value, Forbidden, UnknownResult> => {
+		if (predicate(value)) {
+			throw new TypeError(message ?? typeErrorMessageNot(description, value));
+		}
+	};
+}
+
+export const assertNotUndefined: NotAssertion<undefined, UnknownNotPrimitive<undefined>> = createAssertNot<undefined, UnknownNotPrimitive<undefined>>(isUndefined, 'undefined');
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+export const assertNotNull: NotAssertion<null, UnknownNotPrimitive<null>> = createAssertNot<null, UnknownNotPrimitive<null>>(isNull, 'null');
+// eslint-disable-next-line @typescript-eslint/no-restricted-types
+export const assertNotNullOrUndefined: NotAssertion<null | undefined, UnknownNotPrimitive<null | undefined>>
+	// eslint-disable-next-line @typescript-eslint/no-restricted-types
+	= createAssertNot<null | undefined, UnknownNotPrimitive<null | undefined>>(isNullOrUndefined, 'null or undefined');
+export const assertNotString: NotAssertion<string, UnknownNotPrimitive<string>> = createAssertNot<string, UnknownNotPrimitive<string>>(isString, 'string');
+export const assertNotBoolean: NotAssertion<boolean, UnknownNotPrimitive<boolean>> = createAssertNot<boolean, UnknownNotPrimitive<boolean>>(isBoolean, 'boolean');
+export const assertNotSymbol: NotAssertion<symbol, UnknownNotPrimitive<symbol>> = createAssertNot<symbol, UnknownNotPrimitive<symbol>>(isSymbol, 'symbol');
+export const assertNotBigint: NotAssertion<bigint, UnknownNotPrimitive<bigint>> = createAssertNot<bigint, UnknownNotPrimitive<bigint>>(isBigint, 'bigint');
+export const assertNotPrimitive: NotAssertion<Primitive, object> = createAssertNot<Primitive, object>(isPrimitive, 'primitive'); // eslint-disable-line @typescript-eslint/no-restricted-types
+
+// We intentionally do not support `assert.not(is.undefined, value)`. TypeScript cannot derive safe complement types from arbitrary predicates, and many predicates here are refinements (for example, `is.number` rejects `NaN`). Explicit methods keep runtime checks and type narrowing aligned.
+const notAssertions: NotAssert = {
+	bigint: assertNotBigint,
+	boolean: assertNotBoolean,
+	null: assertNotNull,
+	nullOrUndefined: assertNotNullOrUndefined,
+	primitive: assertNotPrimitive,
+	string: assertNotString,
+	symbol: assertNotSymbol,
+	undefined: assertNotUndefined,
+};
+
 export const assert: Assert = {
 	all: assertAll,
 	any: assertAny,
+	not: notAssertions,
 	optional: assertOptional,
 	array: assertArray,
 	arrayBuffer: assertArrayBuffer,

test/test.ts

@@ -466,6 +466,17 @@ const subClasses = new Map<TypeNameWithFixture, TypeNameWithFixture[]>([
 	['object', keysOf(objectTypes)],
 ]);
 
+const notAssertionFixtures = {
+	bigint: {fixture: 1n, nonFixture: '🦄', typeDescription: 'bigint'},
+	boolean: {fixture: false, nonFixture: '🦄', typeDescription: 'boolean'},
+	null: {fixture: null, nonFixture: '🦄', typeDescription: 'null'},
+	nullOrUndefined: {fixtures: [null, undefined], nonFixture: '🦄', typeDescription: 'null or undefined'},
+	primitive: {fixtures: [false, null, undefined], nonFixture: [], typeDescription: 'primitive'},
+	string: {fixture: '🦄', nonFixture: 1, typeDescription: 'string'},
+	symbol: {fixture: Symbol('🦄'), nonFixture: '🦄', typeDescription: 'symbol'},
+	undefined: {fixture: undefined, nonFixture: null, typeDescription: 'undefined'},
+} as const satisfies Record<keyof typeof isAssert.not, ({fixture: unknown} | {fixtures: unknown[]}) & {nonFixture: unknown; typeDescription: string}>;
+
 // This ensures a certain method matches only the types it's supposed to and none of the other methods' types
 for (const type of keysOf(types)) {
 	test(`is.${type}`, () => {
@@ -2534,6 +2545,14 @@ test('custom assertion message', () => {
 		isAssert.nativePromise(undefined, message);
 	});
 
+	assertThrowsTypeErrorWithMessage(() => {
+		isAssert.not.undefined(undefined, message);
+	});
+
+	assertThrowsTypeErrorWithMessage(() => {
+		isAssert.not.string('hello', message);
+	});
+
 	assertThrowsTypeErrorWithMessage(() => {
 		isAssert.negativeNumber(undefined, message);
 	});
@@ -2715,6 +2734,68 @@ test('custom assertion message', () => {
 	});
 });
 
+test('isAssert.not.undefined', () => {
+	assert.throws(() => {
+		isAssert.not.undefined(undefined);
+	}, {
+		message: 'Expected value which is not `undefined`, received value of type `undefined`.',
+	});
+
+	assert.doesNotThrow(() => {
+		isAssert.not.undefined(null);
+	});
+
+	assert.doesNotThrow(() => {
+		isAssert.not.undefined(false);
+	});
+
+	assert.doesNotThrow(() => {
+		isAssert.not.undefined(0);
+	});
+
+	assert.doesNotThrow(() => {
+		isAssert.not.undefined('');
+	});
+});
+
+test('isAssert.not', () => {
+	assert.deepStrictEqual(new Set(keysOf(isAssert.not)), new Set(keysOf(notAssertionFixtures)));
+
+	for (const type of keysOf(notAssertionFixtures)) {
+		const {nonFixture, typeDescription} = notAssertionFixtures[type];
+		const testAssert = isAssert.not[type];
+		const fixtures = 'fixtures' in notAssertionFixtures[type] ? notAssertionFixtures[type].fixtures : [notAssertionFixtures[type].fixture];
+
+		for (const fixture of fixtures) {
+			assert.throws(() => {
+				testAssert(fixture);
+			}, {
+				message: `Expected value which is not \`${typeDescription}\`, received value of type \`${is(fixture)}\`.`,
+			});
+		}
+
+		assert.doesNotThrow(() => {
+			testAssert(nonFixture);
+		});
+	}
+
+	assert.strictEqual('number' in isAssert.not, false);
+	assert.strictEqual('integer' in isAssert.not, false);
+	assert.strictEqual('object' in isAssert.not, false);
+	assert.strictEqual('blob' in isAssert.not, false);
+	assert.strictEqual('array' in isAssert.not, false);
+	assert.strictEqual('date' in isAssert.not, false);
+	assert.strictEqual('function' in isAssert.not, false);
+	assert.strictEqual('map' in isAssert.not, false);
+	assert.strictEqual('set' in isAssert.not, false);
+});
+
+test('isAssert.not edge cases', () => {
+	assert.doesNotThrow(() => {
+		isAssert.not.null(undefined);
+	});
+});
+
 test('is.optional', () => {
 	assert.ok(is.optional(undefined, is.string));
 	assert.ok(is.optional('🦄', is.string));

test/type-tests.ts

@@ -1,4 +1,10 @@
+import {expectTypeOf} from 'expect-type';
 import is, {
+	assert as isAssert,
+	assertNotNullOrUndefined,
+	assertNotPrimitive,
+	assertNotString,
+	assertNotUndefined,
 	type EvenInteger,
 	type FiniteNumber,
 	type Integer,
@@ -12,10 +18,14 @@ import is, {
 	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`).
@@ -197,6 +207,154 @@ const distinctNumericBrandsStayDistinct = (
 	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);
@@ -218,3 +376,30 @@ 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('🦄');