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# 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)
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