# is

> Type check values

For example, `is.string('🦄') //=> true`

<img src="header.gif" width="182" align="right">

## Highlights

- Written in TypeScript
- [Extensive use of type guards](#type-guards)
- [Supports type assertions](#type-assertions)
- [Aware of generic type parameters](#generic-type-parameters) (use with caution)
- Actively maintained
- ![Millions of downloads per week](https://img.shields.io/npm/dw/@sindresorhus/is)

## Install

```sh
npm install @sindresorhus/is
```

## Usage

```js
import is from '@sindresorhus/is';

is('🦄');
//=> 'string'

is(new Map());
//=> 'Map'

is.number(6);
//=> true
```

[Assertions](#type-assertions) perform the same type checks, but throw an error if the type does not match.

```js
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
import {assert} from '@sindresorhus/is';

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)

Returns the type of `value`.

Primitives are lowercase and object types are camelcase.

Example:

- `'undefined'`
- `'null'`
- `'string'`
- `'symbol'`
- `'Array'`
- `'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 return a boolean for whether the value is of the desired type.

#### Primitives

##### .undefined(value)
##### .null(value)

##### .string(value)
##### .number(value)

Note: `is.number(NaN)` returns `false`. This intentionally deviates from `typeof` behavior to increase user-friendliness of `is` type checks.

##### .boolean(value)
##### .symbol(value)
##### .bigint(value)

#### Built-in types

##### .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.
```

##### .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).

##### .numericString(value)

Returns `true` for a string that represents a number satisfying `is.number`, for example, `'42'` and `'-8.3'`.

Note: `'NaN'` returns `false`, but `'Infinity'` and `'-Infinity'` return `true`.

##### .regExp(value)
##### .date(value)
##### .error(value)
##### .nativePromise(value)
##### .promise(value)

Returns `true` for any object with a `.then()` and `.catch()` method. Prefer this one over `.nativePromise()` as you usually want to allow userland promise implementations too.

##### .generator(value)

Returns `true` for any object that implements its own `.next()` and `.throw()` methods and has a function definition for `Symbol.iterator`.

##### .generatorFunction(value)

##### .asyncFunction(value)

Returns `true` for any `async` function that can be called with the `await` operator.

```js
is.asyncFunction(async () => {});
//=> true

is.asyncFunction(() => {});
//=> false
```

##### .asyncGenerator(value)

```js
is.asyncGenerator(
	(async function * () {
		yield 4;
	})()
);
//=> true

is.asyncGenerator(
	(function * () {
		yield 4;
	})()
);
//=> false
```

##### .asyncGeneratorFunction(value)

```js
is.asyncGeneratorFunction(async function * () {
	yield 4;
});
//=> true

is.asyncGeneratorFunction(function * () {
	yield 4;
});
//=> false
```

##### .boundFunction(value)

Returns `true` for any `bound` function.

```js
is.boundFunction(() => {});
//=> true

is.boundFunction(function () {}.bind(null));
//=> true

is.boundFunction(function () {});
//=> false
```

##### .map(value)
##### .set(value)
##### .weakMap(value)
##### .weakSet(value)
##### .weakRef(value)

#### Typed arrays

##### .int8Array(value)
##### .uint8Array(value)
##### .uint8ClampedArray(value)
##### .int16Array(value)
##### .uint16Array(value)
##### .int32Array(value)
##### .uint32Array(value)
##### .float32Array(value)
##### .float64Array(value)
##### .bigInt64Array(value)
##### .bigUint64Array(value)

#### Structured data

##### .arrayBuffer(value)
##### .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.

##### .nonEmptyStringAndNotWhitespace(value)

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)

Returns `true` if the value is an `Array` and the `.length` is 0.

##### .nonEmptyArray(value)

Returns `true` if the value is an `Array` and the `.length` is more than 0.

##### .emptyObject(value)

Returns `true` if the value is an `Object` and `Object.keys(value).length` is 0.

Please note that `Object.keys` returns only own enumerable properties. Hence something like this can happen:

```js
const object1 = {};

Object.defineProperty(object1, 'property1', {
	value: 42,
	writable: true,
	enumerable: false,
	configurable: true
});

is.emptyObject(object1);
//=> true
```

##### .nonEmptyObject(value)

Returns `true` if the value is an `Object` and `Object.keys(value).length` is more than 0.

##### .emptySet(value)

Returns `true` if the value is a `Set` and the `.size` is 0.

##### .nonEmptySet(Value)

Returns `true` if the value is a `Set` and the `.size` is more than 0.

##### .emptyMap(value)

Returns `true` if the value is a `Map` and the `.size` is 0.

##### .nonEmptyMap(value)

Returns `true` if the value is a `Map` and the `.size` is more than 0.

#### Miscellaneous

##### .directInstanceOf(value, class)

Returns `true` if `value` is a direct instance of `class`.

```js
is.directInstanceOf(new Error(), Error);
//=> true

class UnicornError extends Error {}

is.directInstanceOf(new UnicornError(), Error);
//=> false
```

##### .urlInstance(value)

Returns `true` if `value` is an instance of the [`URL` class](https://developer.mozilla.org/en-US/docs/Web/API/URL).

```js
const url = new URL('https://example.com');

is.urlInstance(url);
//=> true
```

##### .urlString(value)

Returns `true` if `value` is a URL string.

Note: this only does basic checking using the [`URL` class](https://developer.mozilla.org/en-US/docs/Web/API/URL) constructor.

```js
const url = 'https://example.com';

is.urlString(url);
//=> true

is.urlString(new URL(url));
//=> false
```

##### .truthy(value)

Returns `true` for all values that evaluate to true in a boolean context:

```js
is.truthy('🦄');
//=> true

is.truthy(undefined);
//=> false
```

##### .falsy(value)

Returns `true` if `value` is one of: `false`, `0`, `''`, `null`, `undefined`, `NaN`.

##### .nan(value)
##### .nullOrUndefined(value)
##### .primitive(value)

JavaScript primitives are as follows:

- `null`
- `undefined`
- `string`
- `number`
- `boolean`
- `symbol`
- `bigint`

##### .integer(value)

##### .safeInteger(value)

Returns `true` if `value` is a [safe integer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isSafeInteger).

##### .plainObject(value)

An object is plain if it's created by either `{}`, `new Object()`, or `Object.create(null)`.

##### .iterable(value)
##### .asyncIterable(value)
##### .class(value)

Returns `true` if the value is a class constructor.

##### .typedArray(value)

##### .arrayLike(value)

A `value` is array-like if it is not a function and has a `value.length` that is a safe integer greater than or equal to 0.

```js
is.arrayLike(document.forms);
//=> true

function foo() {
	is.arrayLike(arguments);
	//=> true
}
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();
```

##### .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.

##### .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.

```js
is.inRange(3, [0, 5]);
is.inRange(3, [5, 0]);
is.inRange(0, [-2, 2]);
```

##### .inRange(value, upperBound)

Check if `value` (number) is in the range of `0` to `upperBound`.

```js
is.inRange(3, 10);
```

##### .htmlElement(value)

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
import fs from 'node:fs';

is.nodeStream(fs.createReadStream('unicorn.png'));
//=> true
```

##### .observable(value)

Returns `true` if `value` is an `Observable`.

```js
import {Observable} from 'rxjs';

is.observable(new Observable());
//=> true
```

##### .infinite(value)

Check if `value` is `Infinity` or `-Infinity`.

##### .evenInteger(value)

Returns `true` if `value` is an even integer.

##### .oddInteger(value)

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`:

```js
is.any(is.string, {}, true, '🦄');
//=> true

is.any(is.boolean, 'unicorns', [], new Map());
//=> false
```

Using an array of `predicate[]`, returns `true` if **any** of the input `values` returns true for **any** of the `predicates` provided in an array:

```js
is.any([is.string, is.number], {}, true, '🦄');
//=> true

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`:

```js
is.all(is.object, {}, new Map(), new Set());
//=> true

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
```

##### .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.

```ts
import is from '@sindresorhus/is';

const padLeft = (value: string, padding: string | number) => {
	if (is.number(padding)) {
		// `padding` is typed as `number`
		return Array(padding + 1).join(' ') + value;
	}

	if (is.string(padding)) {
		// `padding` is typed as `string`
		return padding + value;
	}

	throw new TypeError(`Expected 'padding' to be of type 'string' or 'number', got '${is(padding)}'.`);
}

padLeft('🦄', 3);
//=> '   🦄'

padLeft('🦄', '🌈');
//=> '🌈🦄'
```

## Type assertions

The type guards are also available as [type assertions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions), which throw an error for unexpected types. It is a convenient one-line version of the often repetitive "if-not-expected-type-throw" pattern.

```ts
import {assert} from '@sindresorhus/is';

const handleMovieRatingApiResponse = (response: unknown) => {
	assert.plainObject(response);
	// `response` is now typed as a plain `object` with `unknown` properties.

	assert.number(response.rating);
	// `response.rating` is now typed as a `number`.

	assert.string(response.title);
	// `response.title` is now typed as a `string`.

	return `${response.title} (${response.rating * 10})`;
};

handleMovieRatingApiResponse({rating: 0.87, title: 'The Matrix'});
//=> 'The Matrix (8.7)'

// This throws an error.
handleMovieRatingApiResponse({rating: '🦄'});
```

### 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.

Use generic type parameters with caution. They are only checked by the TypeScript compiler, and not checked by `is` at runtime. This can lead to unexpected behavior, where the generic type is _assumed_ at compile-time, but actually is something completely different at runtime. It is best to use `unknown` (default) and type-check the value of the generic type parameter at runtime with `is` or `assert`.

```ts
import {assert} from '@sindresorhus/is';

async function badNumberAssumption(input: unknown) {
	// Bad assumption about the generic type parameter fools the compile-time type system.
	assert.promise<number>(input);
	// `input` is a `Promise` but only assumed to be `Promise<number>`.

	const resolved = await input;
	// `resolved` is typed as `number` but was not actually checked at runtime.

	// Multiplication will return NaN if the input promise did not actually contain a number.
	return 2 * resolved;
}

async function goodNumberAssertion(input: unknown) {
	assert.promise(input);
	// `input` is typed as `Promise<unknown>`

	const resolved = await input;
	// `resolved` is typed as `unknown`

	assert.number(resolved);
	// `resolved` is typed as `number`

	// Uses runtime checks so only numbers will reach the multiplication.
	return 2 * resolved;
}

badNumberAssumption(Promise.resolve('An unexpected string'));
//=> NaN

// This correctly throws an error because of the unexpected string value.
goodNumberAssertion(Promise.resolve('An unexpected string'));
```

## FAQ

### Why yet another type checking module?

There are hundreds of type checking modules on npm, unfortunately, I couldn't find any that fit my needs:

- Includes both type methods and ability to get the type
- Types of primitives returned as lowercase and object types as camelcase
- Covers all built-ins
- Unsurprising behavior
- Well-maintained
- Comprehensive test suite

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.

### Why not just use `instanceof` instead of this package?

`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

- [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
- [is-ip](https://github.com/sindresorhus/is-ip) - Check if a string is an IP address
- [is-array-sorted](https://github.com/sindresorhus/is-array-sorted) - Check if an Array is sorted
- [is-error-constructor](https://github.com/sindresorhus/is-error-constructor) - Check if a value is an error constructor
- [is-empty-iterable](https://github.com/sindresorhus/is-empty-iterable) - Check if an Iterable is empty
- [is-blob](https://github.com/sindresorhus/is-blob) - Check if a value is a Blob - File-like object of immutable, raw data
- [has-emoji](https://github.com/sindresorhus/has-emoji) - Check whether a string has any emoji

## Maintainers

- [Sindre Sorhus](https://github.com/sindresorhus)
- [Giora Guttsait](https://github.com/gioragutt)
- [Brandon Smith](https://github.com/brandon93s)
