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Programming language: TypeScript
Tags: Miscellaneous    
Latest version: v3.1.2

@sindresorhus/is alternatives and similar modules

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Type check values

For example, is.string('๐Ÿฆ„') //=> true



$ npm install @sindresorhus/is


const is = require('@sindresorhus/is');

//=> 'string'

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

//=> true

Assertions perform the same type checks, but throw an error if the type does not match.

const {assert} = require('@sindresorhus/is');

//=> Error: Expected value which is `string`, received value of type `number`.

And with TypeScript:

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

// `foo` is now typed as a `string`.



Returns the type of value.

Primitives are lowercase and object types are camelcase.


  • 'undefined'
  • 'null'
  • 'string'
  • 'symbol'
  • 'Array'
  • 'Function'
  • 'Object'

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').


All the below methods accept a value and returns a boolean for whether the value is of the desired type.



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


Built-in types

.array(value, assertion?)

Returns true if value is an array and all of its items match the assertion (if provided).

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.

Keep in mind that functions are objects too.


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.


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.


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


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

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

is.asyncFunction(() => {});
//=> false
    (async function * () {
        yield 4;
//=> true

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

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

Returns true for any bound function.

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

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

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

Typed arrays


Structured data




Returns true if the value is a string and the .length is 0.


Returns true if the value is a string and the .length is more than 0.


Returns true if is.emptyString(value) or if it's a string that is all whitespace.


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


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


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:

const object1 = {};

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

//=> true

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


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


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


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


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


.directInstanceOf(value, class)

Returns true if value is a direct instance of class.

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

class UnicornError extends Error {}

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

Returns true if value is an instance of the URL class.

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

//=> true

Returns true if value is a URL string.

Note: this only does basic checking using the URL class constructor.

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

//=> true

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

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

//=> true

//=> false

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


JavaScript primitives are as follows: null, undefined, string, number, boolean, symbol.


Returns true if value is a safe integer.


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


Returns true for instances created by a class.


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.

//=> true

function foo() {
    //=> true
.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.

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.

is.inRange(3, 10);

Returns true if value is a DOM Element.


Returns true if value is a Node.js stream.

const fs = require('fs');

//=> true

Returns true if value is an Observable.

const {Observable} = require('rxjs');

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

Check if value is Infinity or -Infinity.


Returns true if value is an even integer.


Returns true if value is an odd integer.

.any(predicate | predicate[], ...values)

Using a single predicate argument, returns true if any of the input values returns true in the predicate:

is.any(is.string, {}, true, '๐Ÿฆ„');
//=> 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:

is.any([is.string, is.number], {}, true, '๐Ÿฆ„');
//=> 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:

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

is.all(is.string, '๐Ÿฆ„', [], 'unicorns');
//=> false

Type guards

When using is together with TypeScript, type guards are being used extensively to infer the correct type inside if-else statements.

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('๐Ÿฆ„', 3);
//=> '   ๐Ÿฆ„'

padLeft('๐Ÿฆ„', '๐ŸŒˆ');
//=> '๐ŸŒˆ๐Ÿฆ„'

Type assertions

The type guards are also available as type assertions, which throw an error for unexpected types. It is a convenient one-line version of the often repetitive "if-not-expected-type-throw" pattern.

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

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

    // `response.rating` is now typed as a `number`.

    // `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: '๐Ÿฆ„'});

Generic type parameters

The type guards and type assertions are aware of generic type parameters, 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.

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

async function badNumberAssumption(input: unknown) {
    // Bad assumption about the generic type parameter fools the compile-time type system.
    // `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) {
    // `input` is typed as `Promise<unknown>`

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

    // `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'));


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.

For enterprise

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  • ow - Function argument validation for humans
  • is-stream - Check if something is a Node.js stream
  • is-observable - Check if a value is an Observable
  • file-type - Detect the file type of a Buffer/Uint8Array
  • is-ip - Check if a string is an IP address
  • is-array-sorted - Check if an Array is sorted
  • is-error-constructor - Check if a value is an error constructor
  • is-empty-iterable - Check if an Iterable is empty
  • is-blob - Check if a value is a Blob - File-like object of immutable, raw data
  • has-emoji - Check whether a string has any emoji