is

Type check values

For example, is.string('🦄') //=> true

Highlights

Written in TypeScript
Extensive use of type guards
Actively maintained
2 million weekly downloads

Install

$ npm install @sindresorhus/is

Usage

const is = require('@sindresorhus/is');
 
is('🦄');
//=> 'string'
 
is(new Map());
//=> 'Map'
 
is.number(6);
//=> true

API

is(value)

Returns the type of value.

Primitives are lowercase and object types are camelcase.

Example:

'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').

is.{method}

All the below methods accept a value and returns 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)

.function(value)

.buffer(value)

.object(value)

Keep in mind that functions are objects too.

.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.

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

.boundFunction(value)

Returns true for any bound function.

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

.map(value)

.set(value)

.weakMap(value)

.weakSet(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)

Emptiness

.emptyString(value)

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

.nonEmptyString(value)

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

.emptyStringOrWhitespace(value)

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

.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:

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.

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.

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 constructor.

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:

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.

.integer(value)

.safeInteger(value)

Returns true if value is a safe integer.

.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 for instances created by a class.

.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.

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

.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);

.domElement(value)

Returns true if value is a DOM Element.

.nodeStream(value)

Returns true if value is a Node.js stream.

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

.observable(value)

Returns true if value is an Observable.

const {Observable} = require('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.

.any(predicate, ...values)

Returns true if any of the input values returns true in the predicate:

is.any(is.string, {}, true, '🦄');
//=> true
 
is.any(is.boolean, '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, '🦄', [], '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('🦄', 3);
//=> '   🦄'
 
padLeft('🦄', '🌈');
//=> '🌈🦄'

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.

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