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https://github.com/sindresorhus/is

Type check values
https://github.com/sindresorhus/is

assertions is node-module npm-package type-check type-detection typescript

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Type check values

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# is



> Type check values



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






## 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

```



##### .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

```



##### .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: '🦄'});

```



## 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` and `Map`. 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(input);

	// `input` is a `Promise` but only assumed to be `Promise`.



	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`



	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)