@sindresorhus/is
v7.0.1
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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
- Supports type assertions
- Aware of generic type parameters (use with caution)
- Actively maintained
Install
npm install @sindresorhus/is
Usage
import is from '@sindresorhus/is';
is('🦄');
//=> 'string'
is(new Map());
//=> 'Map'
is.number(6);
//=> true
Assertions perform the same type checks, but throw an error if the type does not match.
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.
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:
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:
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:
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).
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)
.blob(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
.asyncGenerator(value)
is.asyncGenerator(
(async function * () {
yield 4;
})()
);
//=> true
is.asyncGenerator(
(function * () {
yield 4;
})()
);
//=> false
.asyncGeneratorFunction(value)
is.asyncGeneratorFunction(async function * () {
yield 4;
});
//=> true
is.asyncGeneratorFunction(function * () {
yield 4;
});
//=> 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)
.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
.
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.
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:
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
bigint
.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
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.
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.
is.tupleLike([1], [is.number]);
//=> true
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.
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);
.htmlElement(value)
Returns true
if value
is an HTMLElement.
.nodeStream(value)
Returns true
if value
is a Node.js stream.
import fs from 'node:fs';
is.nodeStream(fs.createReadStream('unicorn.png'));
//=> true
.observable(value)
Returns true
if value
is an Observable
.
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.
const data = new FormData();
is.formData(data);
//=> true
.urlSearchParams(value)
Returns true
if value
is an instance of the URLSearchParams
class.
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
:
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:
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
:
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
objects have an internal timestamp value which is the number of milliseconds since the Unix epoch. 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.
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 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 '${is(padding)}'.`);
}
padLeft('🦄', 3);
//=> ' 🦄'
padLeft('🦄', '🌈');
//=> '🌈🦄'
Type assertions
The type guards are also available as type assertions, which throw an error for unexpected types. It is a convenient one-line version of the often repetitive "if-not-expected-type-throw" pattern.
import {assert} from '@sindresorhus/is';
const handleMovieRatingApiResponse = (response: unknown) => {
assert.plainObject(response);
// `response` is now typed as a plain `object` with `unknown` properties.
assert.number(response.rating);
// `response.rating` is now typed as a `number`.
assert.string(response.title);
// `response.title` is now typed as a `string`.
return `${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, 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.
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. Examples of realms are iframes, windows, web workers, and the vm
module in Node.js.
Related
- environment - Check which JavaScript environment your code is running in at runtime
- 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