@lopatnov/get-internal-type

Determine the internal JavaScript [[Class]] of an object. A lightweight, cross-platform TypeScript library for reliable runtime type detection.

Table of Contents

• Supported Types
• Installation
• Usage
• API
• Demo
• Contributing
• Built With
• License

Supported Types

Primitives

| Object Value | Result | Since | | :------------------------- | :------------ | :---- | | undefined | "undefined" | 1.0.0 | | null | "null" | 1.0.0 | | false | "boolean" | 1.0.0 | | 123 | "number" | 1.0.0 | | BigInt(9007199254740991) | "bigint" | 1.0.0 | | "hello" | "string" | 1.0.0 | | Symbol("123") | "symbol" | 1.0.0 |

Functions

| Object Value | Result | Since | | :---------------------- | :-------------------- | :---- | | function(){} | "function" | 1.0.0 | | () => {} | "function" | 1.0.0 | | class Simple {} | "function" | 1.0.0 | | async function(){} | "function" | 2.0.0 | | function* () {} | "generatorfunction" | 1.4.1 | | async function* () {} | "generatorfunction" | 2.0.0 |

Objects

| Object Value | Result | Since | | :------------------------------- | :---------- | :---- | | {} | "object" | 1.0.0 | | new (class SomeCustomClass {}) | "object" | 1.0.0 | | [1,2,3] | "array" | 1.0.0 | | /s+/gi | "regexp" | 1.0.0 | | new Date() | "date" | 1.0.0 | | new Error("A mistake") | "error" | 1.0.0 | | Promise.resolve('any') | "promise" | 1.4.0 |

Collections

| Object Value | Result | Since | | :-------------- | :------ | :---- | | new Map() | "map" | 1.2.0 | | new WeakMap() | "map" | 1.3.1 | | new Set() | "set" | 1.2.0 | | new WeakSet() | "set" | 1.3.1 |

Generators

| Object Value | Result | Since | | :-------------------------- | :------------ | :---- | | (function* () {})() | "generator" | 1.4.1 | | (async function* () {})() | "generator" | 2.0.0 |

Typed Arrays

All typed arrays return "typedarray" (since 1.3.0):

Int8Array, Uint8Array, Uint8ClampedArray, Int16Array, Uint16Array, Int32Array, Uint32Array, Float32Array, Float64Array, BigInt64Array, BigUint64Array

Structured Data

| Object Value | Result | Since | | :------------------------- | :-------------- | :---- | | new ArrayBuffer(8) | "arraybuffer" | 1.5.0 | | new SharedArrayBuffer(8) | "arraybuffer" | 2.0.0 | | new DataView(buffer) | "dataview" | 1.5.0 |

Weak References

| Object Value | Result | Since | | :----------------------------- | :----------------------- | :---- | | new WeakRef(obj) | "weakref" | 2.0.0 | | new FinalizationRegistry(cb) | "finalizationregistry" | 2.0.0 |

Installation

npm install @lopatnov/get-internal-type

Browser:

<script src="https://lopatnov.github.io/get-internal-type/dist/get-internal-type.umd.min.js"></script>

Usage

ES Modules

import getInternalType from "@lopatnov/get-internal-type";
// or named import:
// import { getInternalType } from "@lopatnov/get-internal-type";

getInternalType(42); // "number"
getInternalType("hello"); // "string"
getInternalType([1, 2, 3]); // "array"
getInternalType(new Map()); // "map"
getInternalType(new Set()); // "set"
getInternalType(Promise.resolve()); // "promise"
getInternalType(null); // "null"
getInternalType(undefined); // "undefined"

CommonJS

const getInternalType = require("@lopatnov/get-internal-type");

getInternalType({}); // "object"
getInternalType(new Date()); // "date"
getInternalType(/regex/); // "regexp"
getInternalType(new Error()); // "error"

Browser (UMD)

<script src="https://lopatnov.github.io/get-internal-type/dist/get-internal-type.umd.min.js"></script>
<script>
  console.log(getInternalType([1, 2, 3])); // "array"
</script>

API

getInternalType(obj: any): string

Returns a lowercase string representing the internal [[Class]] of the given object.

| Parameter | Type | Description | | :-------- | :---- | :--------------------------------- | | obj | any | The value to determine the type of |

Returns: string — a lowercase type identifier.

// Primitives
getInternalType(undefined); // "undefined"
getInternalType(null); // "null"
getInternalType(false); // "boolean"
getInternalType(Symbol("123")); // "symbol"
getInternalType(123); // "number"
getInternalType(BigInt(9007199254740991)); // "bigint"
getInternalType("hello"); // "string"

// Objects
getInternalType({}); // "object"
getInternalType([1, 2, 3]); // "array"
getInternalType(/s+/gi); // "regexp"
getInternalType(new Date()); // "date"
getInternalType(new Error("oops")); // "error"

// Collections
getInternalType(new Map()); // "map"
getInternalType(new WeakMap()); // "map"
getInternalType(new Set()); // "set"
getInternalType(new WeakSet()); // "set"

// Functions & Generators
getInternalType(() => {}); // "function"
getInternalType(async () => {}); // "function"
getInternalType(function* () {}); // "generatorfunction"
getInternalType(async function* () {}); // "generatorfunction"
getInternalType((function* () {})()); // "generator"

// Structured Data
getInternalType(Promise.resolve("any")); // "promise"
getInternalType(new ArrayBuffer(16)); // "arraybuffer"
getInternalType(new DataView(new ArrayBuffer(16))); // "dataview"
getInternalType(new Int8Array(8)); // "typedarray"

// Weak References
getInternalType(new WeakRef({})); // "weakref"
getInternalType(new FinalizationRegistry(() => {})); // "finalizationregistry"

Demo

• RunKit demo
• npm.runkit.com

Contributing

Contributions are welcome! Please read CONTRIBUTING.md before opening a pull request.

• Bug reports → open an issue
• Questions → Discussions
• Found it useful? A star on GitHub helps others discover the project

Built With

• TypeScript — strict typing throughout
• Rollup — bundled to ESM, CJS, and UMD formats
• Jest — unit testing with coverage
• ESLint + Prettier — linting and formatting

License

Apache-2.0 © 2019–2026 Oleksandr Lopatnov · LinkedIn