# bigint-factorial

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> Tiny factorial function using JavaScript's built-in [BigInt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt)

Factorials get very big, very quickly. Older packages for calculating them that use the `number` primitive internally suffer the following issues:

- **Subject to imprecision at factorials larger than 18** (at which point they surpass `Number.MAX_SAFE_INTEGER`)
- **Return `Infinity` at factorials larger than 170** (at which point they surpass `Number.MAX_VALUE`)

This package avoids these by dealing exclusively with the `bigint` primitive.

## Install

Install with [npm](https://www.npmjs.com/package/bigint-factorial) (or [Yarn](https://yarnpkg.com/package/bigint-factorial))

```sh
npm i bigint-factorial
```

## Usage

1. Import the package

   ```js
   import factorial from 'bigint-factorial';
   // or
   const factorial = require('bigint-factorial');
   ```

2. Calculate factorials

   ```js
   factorial(5n);
   // ↪︎ 120n

   factorial(6n);
   // ↪︎ 720n

   factorial(7n);
   // ↪︎ 5040n

   factorial(183n);
   // ↪︎ 1211079010624906224171770242040000913194755344907123328387229208384122199143398983962077168073033852647945203036376445283346314711222230177466494273255728793463071956674839497876987299889729720327479783667584731115257659422804284707863129430806869565563037239578516564219715854442393339376435200000000000000000000000000000000000000000000n
   ```

## API

<a name="module_factorial"></a>

### factorial ⇒ <code>bigint</code>
Calculate the factorial of n

**Returns**: <code>bigint</code> - The factorial of n  
**Throws**:

- <code>TypeError</code> If n is not of type 'bigint'
- <code>RangeError</code> If n is negative


| Param | Type | Description |
| --- | --- | --- |
| n | <code>bigint</code> | The number to calculate the factorial of |

## System requirements

- [Node.js](https://nodejs.org/en/) `v10.24.1`
- [npm](https://www.npmjs.com) `v6.14.12`

This package has been tested and confirmed to work on the above versions.

BigInt was [added to Node.js in `v10.4.0`](https://nodejs.org/en/blog/release/v10.4.0/) with [V8 release `v6.7`](https://v8.dev/blog/v8-release-67), therefore this package won't work in Node.js versions earlier than `v10.4.0`.