@phun-ky/typeof

About

A set of JavaScript helper functions to check for types.

Table of Contents

• @phun-ky/typeof
  • About
  • Installation
  • Usage
  • API
    • isBoolean()
    • isBuiltInCallable()
    • isBuiltInConstructor()
    • isClass()
    • isDefined()
    • isInstanceOfUnknownClass()
    • isNotBoolean()
    • isNotNumber()
    • isNotString()
    • isNotUndefined()
    • isNumber()
    • isObjectLoose()
    • isObjectPlain()
    • isObjectStrict()
    • isString()
    • isUndefined()
  • Development
  • Contributing
  • License
  • Changelog
  • Sponsor me

Installation

npm i --save @phun-ky/typeof

Usage

Either import and run the required functions:

import { isString } from '@phun-ky/typeof';

isString('asd'); // true;

API

isBoolean()

Checks if the given value is a boolean.

Param

The value to check.

Call Signature

function isBoolean(value): value is boolean;

Defined in: main.ts:85

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is boolean

Call Signature

function isBoolean(value): boolean;

Defined in: main.ts:90

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isBuiltInCallable()

Checks if a given value is a built-in JavaScript callable.

A built-in callable is either:

• a standard constructor (e.g., Object, Array, Date, Map), or
• a callable non-constructable built-in (BigInt, Symbol).

This function first verifies the value is a function, then tests identity against a curated set of built-ins.

Overloads:

• Predicate: narrows the value to BuiltInCallable on success.
• Boolean: usable in contexts that require a plain (v) => boolean.

Param

The value to check.

Example

isBuiltInCallable(Object); // true
isBuiltInCallable(Array); // true
isBuiltInCallable(BigInt); // true (callable but not a constructor)
isBuiltInCallable(Symbol); // true (callable but not a constructor)
isBuiltInCallable(class X {}); // false
isBuiltInCallable(() => {}); // false
isBuiltInCallable(123); // false

// Type narrowing:
declare const fn: unknown;
if (isBuiltInCallable(fn)) {
  // fn is now typed as BuiltInCallable
  console.log(fn.name);
}

See

• https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global\_Objects
• https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global\_Objects/BigInt
• https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global\_Objects/Symbol

Call Signature

function isBuiltInCallable(value): value is BuiltInCallable;

Defined in: main.ts:539

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is BuiltInCallable

Call Signature

function isBuiltInCallable(value): boolean;

Defined in: main.ts:544

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isBuiltInConstructor()

Checks if a given value is a built-in JavaScript constructor.

This function verifies whether the provided value is a function and matches one of JavaScript's built-in constructors, such as Object, Array, Function, etc.

Param

The value to check.

Example

console.log(isBuiltInConstructor(Object)); // Output: true
console.log(isBuiltInConstructor(Array)); // Output: true
console.log(isBuiltInConstructor(class MyClass {})); // Output: false
console.log(isBuiltInConstructor(() => {})); // Output: false
console.log(isBuiltInConstructor(123)); // Output: false

Call Signature

function isBuiltInConstructor(value): value is BuiltInConstructor;

Defined in: main.ts:439

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is BuiltInConstructor

Call Signature

function isBuiltInConstructor(value): boolean;

Defined in: main.ts:446

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isClass()

Checks if a given value is a class constructor.

This function determines whether the provided value is a class by verifying if it is a function and checking its prototype descriptor. Class constructors always have a non-writeable prototype, while regular functions do not.

Will always return false on built in constructors like Date or Array.

Param

The value to check.

Example

class MyClass {}
console.log(isClass(MyClass)); // Output: true

function regularFunction() {}
console.log(isClass(regularFunction)); // Output: false

console.log(isClass(() => {})); // Output: false
console.log(isClass(null)); // Output: false

Call Signature

function isClass(value): value is ClassCtor<any>;

Defined in: main.ts:364

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is ClassCtor<any>

Call Signature

function isClass(value): boolean;

Defined in: main.ts:369

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isDefined()

Copy of isNotUndefined

Param

The value to check.

Call Signature

function isDefined<T>(value): value is Exclude<T, undefined>;

Defined in: main.ts:165

Type Parameters

| Type Parameter | | -------------- | | T |

Parameters

| Parameter | Type | | --------- | ---- | | value | T |

Returns

value is Exclude<T, undefined>

Call Signature

function isDefined(value): boolean;

Defined in: main.ts:170

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isInstanceOfUnknownClass()

Checks if a given value is an instance of a non-standard (unknown) class.

This function determines whether the provided value is an object and has a prototype that is neither Object.prototype (standard object) nor null (no prototype). It helps differentiate between instances of custom classes and plain objects.

Param

The value to check.

Example

class MyClass {}
console.log(isInstanceOfUnknownClass(new MyClass())); // Output: true
console.log(isInstanceOfUnknownClass({})); // Output: false
console.log(isInstanceOfUnknownClass(Object.create(null))); // Output: false
console.log(isInstanceOfUnknownClass([])); // Output: true

Call Signature

function isInstanceOfUnknownClass(value): value is object;

Defined in: main.ts:595

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is object

Call Signature

function isInstanceOfUnknownClass(value): boolean;

Defined in: main.ts:600

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isNotBoolean()

Checks if the given value is not a boolean.

Param

The value to check.

Call Signature

function isNotBoolean<T>(value): value is Exclude<T, boolean>;

Defined in: main.ts:105

Type Parameters

| Type Parameter | | -------------- | | T |

Parameters

| Parameter | Type | | --------- | ---- | | value | T |

Returns

value is Exclude<T, boolean>

Call Signature

function isNotBoolean(value): boolean;

Defined in: main.ts:110

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isNotNumber()

Checks if the given value is not a number.

Param

The value to check.

Call Signature

function isNotNumber<T>(value): value is Exclude<T, number>;

Defined in: main.ts:65

Type Parameters

| Type Parameter | | -------------- | | T |

Parameters

| Parameter | Type | | --------- | ---- | | value | T |

Returns

value is Exclude<T, number>

Call Signature

function isNotNumber(value): boolean;

Defined in: main.ts:70

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isNotString()

Checks if the given value is not a string.

Param

The value to check.

Call Signature

function isNotString<T>(value): value is Exclude<T, string>;

Defined in: main.ts:25

Type Parameters

| Type Parameter | | -------------- | | T |

Parameters

| Parameter | Type | | --------- | ---- | | value | T |

Returns

value is Exclude<T, string>

Call Signature

function isNotString(value): boolean;

Defined in: main.ts:30

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isNotUndefined()

Checks if the given value is not undefined.

Param

The value to check.

Call Signature

function isNotUndefined<T>(value): value is Exclude<T, undefined>;

Defined in: main.ts:145

Type Parameters

| Type Parameter | | -------------- | | T |

Parameters

| Parameter | Type | | --------- | ---- | | value | T |

Returns

value is Exclude<T, undefined>

Call Signature

function isNotUndefined(value): boolean;

Defined in: main.ts:150

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isNumber()

Checks if the given value is a number.

Param

The value to check.

Call Signature

function isNumber(value): value is number;

Defined in: main.ts:45

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is number

Call Signature

function isNumber(value): boolean;

Defined in: main.ts:50

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isObjectLoose()

Checks if a given value is an object or a function.

This function verifies whether the provided value is of type 'object' or 'function' while ensuring that null is excluded.

Param

The value to check.

Example

console.log(isObjectLoose({})); // Output: true
console.log(isObjectLoose([])); // Output: true
console.log(isObjectLoose(() => {})); // Output: true
console.log(isObjectLoose(null)); // Output: false
console.log(isObjectLoose(42)); // Output: false

Features

• ✅ Recognizes all objects (plain objects, arrays, functions, dates, etc.).
• ✅ Recognizes functions as objects (since functions are technically objects in JavaScript).
• ❌ Does not differentiate between plain objects and special objects (like arrays, functions, DOM nodes, etc.).

Behaviour

• ✅ isObjectLoose({}) → true
• ✅ isObjectLoose([]) → true
• ✅ isObjectLoose(() => {}) → true
• ❌ isObjectLoose(null) → false

When to use

• Use isObjectStrict when you need a strict check for plain objects.
• Use isObjectLoose if you need to check if a value is an object-like structure, including functions.

Comparison

| Feature | Strict Check (isObjectStrict) | Loose Check (isObjectLoose) | | ---------------------------------------- | ------------------------------- | ----------------------------- | | Recognizes plain objects | ✅ Yes | ✅ Yes | | Recognizes functions | ❌ No | ✅ Yes | | Recognizes arrays | ❌ No | ✅ Yes | | Recognizes Object.create(null) objects | ✅ Yes | ✅ Yes | | Recognizes class instances | ❌ No | ✅ Yes | | Recognizes DOM elements | ❌ No | ✅ Yes | | Complexity | 🔴 High | 🟢 Low |

Call Signature

function isObjectLoose(value): value is object;

Defined in: main.ts:301

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is object

Call Signature

function isObjectLoose(value): boolean;

Defined in: main.ts:306

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isObjectPlain()

Determines whether a value is a plain object (i.e., created via an object literal, Object.create(null), or with Object as its prototype).

This excludes arrays, functions, class instances, built-ins like Date/Map/Set, and other exotic objects.

Param

The value to test.

Example

const a: unknown = { x: 1 };
const b: unknown = [];
const c: unknown = new Date();
const d: unknown = Object.create(null);

isObjectPlain(a); // true
isObjectPlain(b); // false (array)
isObjectPlain(c); // false (built-in)
isObjectPlain(d); // true (null prototype)

// Type narrowing example:
const value: unknown = { foo: 42 };
if (isObjectPlain(value)) {
  // value is now Record<string, unknown>
  console.log(value.foo);
}

See

• https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global\_Objects/Object/toString
• https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global\_Objects/Object/getPrototypeOf

Call Signature

function isObjectPlain(value): value is Record<string, unknown>;

Defined in: main.ts:185

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is Record<string, unknown>

Call Signature

function isObjectPlain(value): boolean;

Defined in: main.ts:190

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isObjectStrict()

Checks if a given value is a plain object.

A plain object is an object created by the {} syntax, Object.create(null), or using new Object(). This function ensures that the value is an object and does not have an unusual prototype chain.

Param

The value to check.

Example

console.log(isObjectStrict({})); // Output: true
console.log(isObjectStrict(Object.create(null))); // Output: true
console.log(isObjectStrict([])); // Output: false
console.log(isObjectStrict(new Date())); // Output: false
console.log(isObjectStrict(null)); // Output: false

Features

• ✅ Recognizes only plain objects (created via {}, new Object(), Object.create(null), etc.).
• ❌ Rejects arrays, functions, DOM elements, class instances, and custom objects with modified constructors.

Behaviour

• ✅ isObjectStrict({}) → true
• ❌ isObjectStrict([]) → false
• ❌ isObjectStrict(() => {}) → false
• ✅ isObjectStrict(Object.create(null)) → true

When to use

• Use isObjectStrict when you need a strict check for plain objects.
• Use isObjectLoose if you need to check if a value is an object-like structure, including functions.

Call Signature

function isObjectStrict(value): value is Record<string, unknown>;

Defined in: main.ts:236

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is Record<string, unknown>

Call Signature

function isObjectStrict(value): boolean;

Defined in: main.ts:243

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isString()

Checks if the given value is a string.

Param

The value to check.

Call Signature

function isString(value): value is string;

Defined in: main.ts:5

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is string

Call Signature

function isString(value): boolean;

Defined in: main.ts:10

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

isUndefined()

Checks if the given value is undefined.

Param

The value to check.

Call Signature

function isUndefined(value): value is undefined;

Defined in: main.ts:125

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

value is undefined

Call Signature

function isUndefined(value): boolean;

Defined in: main.ts:130

Parameters

| Parameter | Type | | --------- | --------- | | value | unknown |

Returns

boolean

Development

// Build
$ npm run build
// Run dev
$ npm run dev
// Test
$ npm test

Contributing

Want to contribute? Please read the CONTRIBUTING.md and CODE_OF_CONDUCT.md

License

This project is licensed under the MIT License - see the LICENSE file for details.

Changelog

See the CHANGELOG.md for details on the latest updates.

Sponsor me

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The sponsorship is an unique opportunity to alleviate more hours for me to maintain my projects, create new ones and contribute to the large community we're all part of :)

Support me on GitHub Sponsors.

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