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# is [](https://travis-ci.org/sindresorhus/is) > Type check values: `is.string('🦄') //=> true` <img src="header.gif" width="182" align="right"> ## Install ``` $ npm install @sindresorhus/is ``` ## Usage ```js const is = require('@sindresorhus/is'); is('🦄'); //=> 'string' is(new Map()); //=> 'Map' is.number(6); //=> true ``` 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 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('🦄', '🌈'); //=> '🌈🦄' ``` ## 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) ##### .boolean(value) ##### .symbol(value) #### Built-in types ##### .array(value) ##### .function(value) ##### .buffer(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. For example, `'42'` and `'-8'`. 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 ``` ##### .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) #### Typed arrays ##### .int8Array(value) ##### .uint8Array(value) ##### .uint8ClampedArray(value) ##### .int16Array(value) ##### .uint16Array(value) ##### .int32Array(value) ##### .uint32Array(value) ##### .float32Array(value) ##### .float64Array(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: ```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 ``` ### .url(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.url(url); //=> true is.url(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`. ##### .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` 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. ```js 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. ```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); ``` ##### .domElement(value) Returns `true` if `value` is a DOM Element. ##### .nodeStream(value) Returns `true` if `value` is a Node.js [stream](https://nodejs.org/api/stream.html). ```js const fs = require('fs'); is.nodeStream(fs.createReadStream('unicorn.png')); //=> true ``` ##### .observable(value) Returns `true` if `value` is an `Observable`. ```js const {Observable} = require('rxjs'); is.observable(new Observable()); //=> true ``` ##### .infinite(value) Check if `value` is `Infinity` or `-Infinity`. ##### .even(value) Returns `true` if `value` is an even integer. ##### .odd(value) Returns `true`