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https://github.com/dankogai/js-combinatorics
power set, combination, permutation and more in JavaScript
https://github.com/dankogai/js-combinatorics
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power set, combination, permutation and more in JavaScript
- Host: GitHub
- URL: https://github.com/dankogai/js-combinatorics
- Owner: dankogai
- License: mit
- Created: 2013-03-07T15:54:02.000Z (almost 12 years ago)
- Default Branch: main
- Last Pushed: 2024-02-21T14:50:05.000Z (11 months ago)
- Last Synced: 2024-05-21T22:04:19.748Z (8 months ago)
- Language: JavaScript
- Homepage:
- Size: 326 KB
- Stars: 742
- Watchers: 36
- Forks: 98
- Open Issues: 4
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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js-combinatorics
================
Simple combinatorics in JavaScript
## HEADS UP: Version 2 and BigInt
Now that [Internet Explorer has officially retired], It is safe to assume `BigInt` is available in every JavaScript environment. From version 2.0 this module goes fully BigInt. While integer arguments can still be either `number` or `bigint`, all integer values that can be `bigint` are always `bigint`, whereas previous versions may return `number` when the value <= `Number.MAX_SAFE_INTEGER`. It is not only more combinatorically natural, but also makes debugging easier especially on TypeScript.
[Internet Explorer has officially retired]: https://blogs.windows.com/windowsexperience/2022/06/15/internet-explorer-11-has-retired-and-is-officially-out-of-support-what-you-need-to-know/
[in every JavaScript environment]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt
### For Swift programmers
Check [swift-combinatorics]. More naturally implemented with generics and protocol.
[swift-combinatorics]: https://github.com/dankogai/swift-combinatorics
## SYNOPSIS
```javascript
import * as $C from './combinatorics.js';
let it = new $C.Combination('abcdefgh', 4);
for (const elem of it) {
console.log(elem) // ['a', 'b', 'c', 'd'] ... ['e', 'f', 'g', 'h']
}
```
## Usage
load everything…
```javascript
import * as Combinatorics from './combinatorics.js';
```
or just objects you want.
```javascript
import { Combination, Permutation } from './combinatorics.js';
```
You don't even have to install if you `import` from CDNs.
```javascript
import * as $C from 'https://cdn.jsdelivr.net/npm/[email protected]/combinatorics.min.js';
```
Since this is an ES6 module, `type="module"` is required the `` tags. of your HTML files. But you can make it globally available as follows.
```html
<script type="module">
import * as $C from 'combinatorics.js';
window.Combinatorics = $C;
// now you can access Combinatorics
let c = new Combinatorics.Combination('abcdefgh', 4);
```
### node.js REPL
```shell
% node
Welcome to Node.js v16.15.0.
Type ".help" for more information.
> const $C = await import('js-combinatorics')
undefined
> $C
[Module: null prototype] {
BaseN: [class BaseN extends _CBase],
CartesianProduct: [class CartesianProduct extends _CBase],
Combination: [class Combination extends _CBase],
Permutation: [class Permutation extends _CBase],
PowerSet: [class PowerSet extends _CBase],
combinadic: [Function: combinadic],
combination: [Function: combination],
factoradic: [Function: factoradic],
factorial: [Function: factorial],
permutation: [Function: permutation],
randomInteger: [Function: randomInteger],
version: '2.1.2'
}
> [...new $C.Permutation('abcd')]
[
[ 'a', 'b', 'c', 'd' ], [ 'a', 'b', 'd', 'c' ],
[ 'a', 'c', 'b', 'd' ], [ 'a', 'c', 'd', 'b' ],
[ 'a', 'd', 'b', 'c' ], [ 'a', 'd', 'c', 'b' ],
[ 'b', 'a', 'c', 'd' ], [ 'b', 'a', 'd', 'c' ],
[ 'b', 'c', 'a', 'd' ], [ 'b', 'c', 'd', 'a' ],
[ 'b', 'd', 'a', 'c' ], [ 'b', 'd', 'c', 'a' ],
[ 'c', 'a', 'b', 'd' ], [ 'c', 'a', 'd', 'b' ],
[ 'c', 'b', 'a', 'd' ], [ 'c', 'b', 'd', 'a' ],
[ 'c', 'd', 'a', 'b' ], [ 'c', 'd', 'b', 'a' ],
[ 'd', 'a', 'b', 'c' ], [ 'd', 'a', 'c', 'b' ],
[ 'd', 'b', 'a', 'c' ], [ 'd', 'b', 'c', 'a' ],
[ 'd', 'c', 'a', 'b' ], [ 'd', 'c', 'b', 'a' ]
]
>
```
### commonjs (node.js)
`./combinatorics.js` is an ECMAScript module but if you still need a UMD or commonjs version, they are available as `./umd/combinatorics.js` and `./commonjs/combinatorics.js` respectively.
## Description
### Arithmetic Functions
Self-explanatory, are they not?
```javascript
import { permutation, combination, factorial, randomInteger } from './combinatorics.js';
permutation(24, 12); // 1295295050649600n
permutation(26, 13); // 64764752532480000n
combination(56, 28); // 7648690600760440n
combination(58, 29); // 30067266499541040n
factorial(18); // 6402373705728000n
factorial(19); // 121645100408832000n
randomInteger(6402373705727999); // random n [0,6402373705728000)
randomInteger(121645100408832000n); // ramdom n [0n, 121645100408832000n)
```
The arithmetic functions above accept both `Number` and `BigInt` (if supported). Return answers always in `BigInt`.
#### `factoradic()` and `combinadic()`
They need a little more explanation.
```javascript
import { factoradic, combinadic } from './combinatorics.js';
factoradic(6402373705727999); // [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
factoradic(121645100408831999n); // [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]
const c16_8 = combinadic(16, 8);
c16_8(0); // [ 0, 1, 2, 3, 4, 5, 6, 7]
c16_8(12870); // [ 8, 9, 10, 11, 12, 13, 14, 15]
const c58_29 = combinadic(58, 29);
c58_29(0); /* [
0, 1, 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20, 21, 22, 23, 24, 25, 26,
27, 28
] */
c58_29(30067266499541039n); /* [
29, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39, 40, 41, 42, 43, 44, 45, 46,
47, 48, 49, 50, 51, 52, 53, 54, 55,
56, 57
] */
```
`factoradic(n)` returns the [factoradic] representation of `n`. For an array `ary` with `n` elements, you can get its `n`th permutation by picking `ary[i]` for each `i` in the factoradic.
[factoradic]: https://en.wikipedia.org/wiki/Factorial_number_system
Unlike other arithmetic functions, `combinadic()` returns a function which returns `m`th [combinadic] digit of `n C k`. For an array `ary` with `n` elements, you can get its `m`th combination by picking `ary[i]` for each `i` in the combinadic.
[combinadic]: https://en.wikipedia.org/wiki/Combinatorial_number_system
### classes
The module comes with `Permutation`, `Combination`, `PowerSet`, `BaseN`, and `CartesianProduct`. You can individually `import` them or all of them via `import *`
```javascript
import * as $C from 'combinatorics.js';
```
You construct an iterable object by giving a seed iterable and options. in the example below, `'abcdefgh'` is the seed and `4` is the size of the element.
```javascript
let it = new $C.Combination('abcdefgh', 4);
```
if you hate `new`, you can use `Klass.of` where `Klass` is one of the classes this module offers.
```javascript
let it = $C.Combination.of('abcdefgh', 4);
```
Once constructed, you can iterate via `for … of` statement or turn it into an array via `[...]` construct.
```javascript
[...it]; /* [
[ 'a', 'b', 'c', 'd' ], [ 'a', 'b', 'c', 'e' ], [ 'a', 'b', 'c', 'f' ],
[ 'a', 'b', 'c', 'g' ], [ 'a', 'b', 'c', 'h' ], [ 'a', 'b', 'd', 'e' ],
[ 'a', 'b', 'd', 'f' ], [ 'a', 'b', 'd', 'g' ], [ 'a', 'b', 'd', 'h' ],
[ 'a', 'b', 'e', 'f' ], [ 'a', 'b', 'e', 'g' ], [ 'a', 'b', 'e', 'h' ],
[ 'a', 'b', 'f', 'g' ], [ 'a', 'b', 'f', 'h' ], [ 'a', 'b', 'g', 'h' ],
[ 'a', 'c', 'd', 'e' ], [ 'a', 'c', 'd', 'f' ], [ 'a', 'c', 'd', 'g' ],
[ 'a', 'c', 'd', 'h' ], [ 'a', 'c', 'e', 'f' ], [ 'a', 'c', 'e', 'g' ],
[ 'a', 'c', 'e', 'h' ], [ 'a', 'c', 'f', 'g' ], [ 'a', 'c', 'f', 'h' ],
[ 'a', 'c', 'g', 'h' ], [ 'a', 'd', 'e', 'f' ], [ 'a', 'd', 'e', 'g' ],
[ 'a', 'd', 'e', 'h' ], [ 'a', 'd', 'f', 'g' ], [ 'a', 'd', 'f', 'h' ],
[ 'a', 'd', 'g', 'h' ], [ 'a', 'e', 'f', 'g' ], [ 'a', 'e', 'f', 'h' ],
[ 'a', 'e', 'g', 'h' ], [ 'a', 'f', 'g', 'h' ], [ 'b', 'c', 'd', 'e' ],
[ 'b', 'c', 'd', 'f' ], [ 'b', 'c', 'd', 'g' ], [ 'b', 'c', 'd', 'h' ],
[ 'b', 'c', 'e', 'f' ], [ 'b', 'c', 'e', 'g' ], [ 'b', 'c', 'e', 'h' ],
[ 'b', 'c', 'f', 'g' ], [ 'b', 'c', 'f', 'h' ], [ 'b', 'c', 'g', 'h' ],
[ 'b', 'd', 'e', 'f' ], [ 'b', 'd', 'e', 'g' ], [ 'b', 'd', 'e', 'h' ],
[ 'b', 'd', 'f', 'g' ], [ 'b', 'd', 'f', 'h' ], [ 'b', 'd', 'g', 'h' ],
[ 'b', 'e', 'f', 'g' ], [ 'b', 'e', 'f', 'h' ], [ 'b', 'e', 'g', 'h' ],
[ 'b', 'f', 'g', 'h' ], [ 'c', 'd', 'e', 'f' ], [ 'c', 'd', 'e', 'g' ],
[ 'c', 'd', 'e', 'h' ], [ 'c', 'd', 'f', 'g' ], [ 'c', 'd', 'f', 'h' ],
[ 'c', 'd', 'g', 'h' ], [ 'c', 'e', 'f', 'g' ], [ 'c', 'e', 'f', 'h' ],
[ 'c', 'e', 'g', 'h' ], [ 'c', 'f', 'g', 'h' ], [ 'd', 'e', 'f', 'g' ],
[ 'd', 'e', 'f', 'h' ], [ 'd', 'e', 'g', 'h' ], [ 'd', 'f', 'g', 'h' ],
[ 'e', 'f', 'g', 'h' ]
] */
```
#### `.length`
The object has `.length` so you don't have to iterate to count the elements. Note the value is in `bigint` so you may need to convert to `number`.
```javascript
it.length // 70n
[...it].length // 70
it.length == [...it].length // true because comparisons work between number and bigint
it.length === [...it].length // false because types are different
```
#### `.at()` (or `.nth()`)
And the object has `.at(n)` method so you can random-access each element. This is the equivalent of subscript in `Array`. It was previously named `.nth()` but it was renamed to `.at()` ala `Array.prototype.at()` in ES2020. `.nth()` still available for backward compatibility.
```javascript
it.at(0); // [ 'a', 'b', 'c', 'd' ];
it.at(69); // [ 'a', 'd', 'c', 'h' ];
```
`at()` accepts both `Number` and `BigInt`.
```javascript
it.at(69n); // [ 'a', 'd', 'c', 'h' ];
```
`at()` also accepts negative indexes. In which case `n` is `(-n)th` element from `.length`.
```javascript
it.at(-1); // [ 'a', 'd', 'c', 'h' ]
it.at(-70); // [ 'a', 'b', 'c', 'd' ]
```
#### `.sample()`
And `.sample()` picks random element, which is defined as `.at(randomInteger(.length))`.
```javascript
it.sample() // one of ['a', 'b', 'c', 'd'] ... ['a', 'd', 'e', 'f']
```
### Beyond `Number.MAX_SAFE_INTEGER`
Occasionally you need `BigInt` to access elements beyond `Number.MAX_SAFE_INTEGER`.
```javascript
it = new $C.Permutation('abcdefghijklmnopqrstuvwxyz');
it.length; // 403291461126605635584000000n
```
You can still access elements before `Number.MAX_SAFE_INTEGER` in `Number`.
```javascript
it.at(0); /* [
'a', 'b', 'c', 'd', 'e', 'f',
'g', 'h', 'i', 'j', 'k', 'l',
'm', 'n', 'o', 'p', 'q', 'r',
's', 't', 'u', 'v', 'w', 'x',
'y', 'z'
] */
it.at(9007199254740990); /* [
'a', 'b', 'c', 'd', 'e', 'f',
'g', 'i', 'p', 'n', 'r', 'z',
'm', 'h', 'y', 'x', 'u', 't',
'l', 'j', 'k', 'q', 's', 'o',
'v', 'w'
] */
```
But how are you goint to acccess elements beyond that? Just use `BigInt`.
```javascript
it.at(9007199254740991n); /* [
'a', 'b', 'c', 'd', 'e', 'f',
'g', 'i', 'p', 'n', 'r', 'z',
'm', 'h', 'y', 'x', 'u', 't',
'l', 'j', 'k', 'q', 's', 'o',
'w', 'v'
] */
it.at(it.length - 1n); /* [
'z', 'y', 'x', 'w', 'v', 'u',
't', 's', 'r', 'q', 'p', 'o',
'n', 'm', 'l', 'k', 'j', 'i',
'h', 'g', 'f', 'e', 'd', 'c',
'b', 'a'
] */
```
You can tell if you need `BigInt` via `.isBig`. Note `.length` is always `bigint` from version 2.0 so you may not need this method any more. So it is now deprecated.
```javascript
new $C.Permutation('0123456789').isBig; // false
new $C.Permutation('abcdefghijklmnopqrstuvwxyz').isBig; // true
```
You can also check if it is safe on your platform via `.isSafe`. It is now deprecated for the same reason as `.isBig`.
```javascript
new $C.Permutation('abcdefghijklmnopqrstuvwxyz').isSafe; // always true
```
### class `Permutation`
An iterable which permutes a given iterable.
`new Permutation(seed, size)`
* `seed`: the seed iterable. `[...seed]` becomes the seed array.
* `size`: the number of elements in the iterated element. defaults to `seed.length`
````javascript
import {Permutation} from './combinatorics.js';
let it = new Permutation('abcd'); // size 4 is assumed
it.length; // 24n
[...it]; /* [
[ 'a', 'b', 'c', 'd' ], [ 'a', 'b', 'd', 'c' ],
[ 'a', 'c', 'b', 'd' ], [ 'a', 'c', 'd', 'b' ],
[ 'a', 'd', 'b', 'c' ], [ 'a', 'd', 'c', 'b' ],
[ 'b', 'a', 'c', 'd' ], [ 'b', 'a', 'd', 'c' ],
[ 'b', 'c', 'a', 'd' ], [ 'b', 'c', 'd', 'a' ],
[ 'b', 'd', 'a', 'c' ], [ 'b', 'd', 'c', 'a' ],
[ 'c', 'a', 'b', 'd' ], [ 'c', 'a', 'd', 'b' ],
[ 'c', 'b', 'a', 'd' ], [ 'c', 'b', 'd', 'a' ],
[ 'c', 'd', 'a', 'b' ], [ 'c', 'd', 'b', 'a' ],
[ 'd', 'a', 'b', 'c' ], [ 'd', 'a', 'c', 'b' ],
[ 'd', 'b', 'a', 'c' ], [ 'd', 'b', 'c', 'a' ],
[ 'd', 'c', 'a', 'b' ], [ 'd', 'c', 'b', 'a' ]
] */
it = new Permutation('abcdefghijklmnopqrstuvwxyz0123456789');
it.length; // 371993326789901217467999448150835200000000n
it.at(371993326789901217467999448150835199999999n); /* [
'9', '8', '7', '6', '5', '4', '3',
'2', '1', '0', 'z', 'y', 'x', 'w',
'v', 'u', 't', 's', 'r', 'q', 'p',
'o', 'n', 'm', 'l', 'k', 'j', 'i',
'h', 'g', 'f', 'e', 'd', 'c', 'b',
'a'
] */
````
Making a permutation of the iterable then taking its sample is functionally the same as [Fisher–Yates shuffle] of the iterable. Instead of shuffling the deck, it make all possible cases available and let you pick one.
```javascript
it.sample(); // something between ['a','b', ... '9'] and ['9','8',....'a']
```
It is in fact a little better because `.sample()` only needs one random number (between 0 and `.length - 1`) while Fisher–Yates needs `n` random numbers.
[Fisher–Yates shuffle]: https://en.wikipedia.org/wiki/Fisher–Yates_shuffle
### class `Combination`
An iterable which emits a combination of a given iterable.
`new Combination(seed, size)`
* `seed`: the seed iterable.
* `size`: the number of elements in the iterated element.
````javascript
import {Combination} from './combinatorics.js';
let it = new Combination('abcd', 2);
it.length; // 6n
[...it]; /* [
[ 'a', 'b' ],
[ 'a', 'c' ],
[ 'a', 'd' ],
[ 'b', 'c' ],
[ 'b', 'd' ],
[ 'c', 'd' ]
] */
let a100 = Array(100).fill(0).map((v,i)=>i); // [0, 1, ...99]
it = new Combination(a100, 50);
it.length; // 100891344545564193334812497256n
it.at(100891344545564193334812497255n); /* [
50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71,
72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82,
83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93,
94, 95, 96, 97, 98, 99
] */
````
### class `PowerSet`
An iterable which emits each element of its power set.
`new PowerSet(seed)`
* `seed`: the seed iterable.
````javascript
import {PowerSet} from './combinatorics.js';
let it = new PowerSet('abc');
it.length; // 8n
[...it]; /* [
[],
[ 'a' ],
[ 'b' ],
[ 'a', 'b' ],
[ 'c' ],
[ 'a', 'c' ],
[ 'b', 'c' ],
[ 'a', 'b', 'c' ]
] */
it = new PowerSet(
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
);
it.length; // 18446744073709551616n
it.at(18446744073709551615n); /* [
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I',
'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a',
'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1',
'2', '3', '4', '5', '6', '7', '8', '9', '+',
'/'
] */
````
### class `BaseN`
An iterable which emits all numbers in the given system.
`new BaseN(seed, size)`
* `seed`: the seed iterable whose elements represent digits.
* `size`: the number of digits
```javascript
import {BaseN} from './combinatorics.js';
let it = new BaseN('abc', 3);
it.length; // 27n
[...it]; /* [
[ 'a', 'a', 'a' ], [ 'b', 'a', 'a' ],
[ 'c', 'a', 'a' ], [ 'a', 'b', 'a' ],
[ 'b', 'b', 'a' ], [ 'c', 'b', 'a' ],
[ 'a', 'c', 'a' ], [ 'b', 'c', 'a' ],
[ 'c', 'c', 'a' ], [ 'a', 'a', 'b' ],
[ 'b', 'a', 'b' ], [ 'c', 'a', 'b' ],
[ 'a', 'b', 'b' ], [ 'b', 'b', 'b' ],
[ 'c', 'b', 'b' ], [ 'a', 'c', 'b' ],
[ 'b', 'c', 'b' ], [ 'c', 'c', 'b' ],
[ 'a', 'a', 'c' ], [ 'b', 'a', 'c' ],
[ 'c', 'a', 'c' ], [ 'a', 'b', 'c' ],
[ 'b', 'b', 'c' ], [ 'c', 'b', 'c' ],
[ 'a', 'c', 'c' ], [ 'b', 'c', 'c' ],
[ 'c', 'c', 'c' ]
] */
it = BaseN('0123456789abcdef', 16);
it.length; // 18446744073709551616n
it.at(18446744073709551615n); /* [
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f'
] */
```
### class `CartesianProduct`
A [cartesian product] of given sets.
[cartesian Product]: https://en.wikipedia.org/wiki/Cartesian_product
`new CartesianProduct(...args)`
* `args`: iterables that represent sets
```javascript
import {CartesianProduct} from './combinatorics.js';
let it = new CartesianProduct('012','abc','xyz');
it.length; // 27n
[...it]; /* [
[ '0', 'a', 'x' ], [ '1', 'a', 'x' ],
[ '2', 'a', 'x' ], [ '0', 'b', 'x' ],
[ '1', 'b', 'x' ], [ '2', 'b', 'x' ],
[ '0', 'c', 'x' ], [ '1', 'c', 'x' ],
[ '2', 'c', 'x' ], [ '0', 'a', 'y' ],
[ '1', 'a', 'y' ], [ '2', 'a', 'y' ],
[ '0', 'b', 'y' ], [ '1', 'b', 'y' ],
[ '2', 'b', 'y' ], [ '0', 'c', 'y' ],
[ '1', 'c', 'y' ], [ '2', 'c', 'y' ],
[ '0', 'a', 'z' ], [ '1', 'a', 'z' ],
[ '2', 'a', 'z' ], [ '0', 'b', 'z' ],
[ '1', 'b', 'z' ], [ '2', 'b', 'z' ],
[ '0', 'c', 'z' ], [ '1', 'c', 'z' ],
[ '2', 'c', 'z' ]
] */
```
Since the number of arguments to `CartesianProduct` is variable, it is sometimes helpful to give a single array with all arguments. But you cannot `new ctor.apply(null, args)` this case. To mitigate that, you can use `.from()`.
```javascript
let a16 = Array(16).fill('0123456789abcdef');
it = CartesianProduct.from(a16);
it.length; // 18446744073709551616n
it.at(18446744073709551615n); /* [
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f',
'f', 'f', 'f', 'f'
] */
````
## What's new from version 0.x?
`js-combinatorics` has gone ES2015 since version 1.
* native iterator instead of custom
* module. `import` instead of `require`.
* `BigInt` where possible
And from version 1.2 it is written in TypeScript. `combinatorics.js` and `combinatorics.d.ts` are compiled from `combinatorics.ts`.
APIs will change accordingly. Old versions are available in the `version0` branch.
### What's gone from version 0.x?
* `bigCombination` is gone because all classes now can handle big -- combinatorially big! -- cases thanks to [BigInt] support getting standard. Safari 13 and below is a major exception but BigInt is coming to Safari 14 and up.
[BigInt]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt
* `permutationCombination` is gone because the name is misleading and it is now trivially easy to reconstruct as follow:
```javascript
class permutationCombination {
constructor(seed) {
this.seed = [...seed];
}
[Symbol.iterator]() {
return function*(it){
for (let i = 1, l = it.length; i <= l; i++) {
yield* new Permutation(it, i);
}
}(this.seed);
}
}
```
* `js-combinatorics` is now natively iterable. Meaning its custom iterators are gone -- with its methods like `.map` and `.filter`. JS iterators are very minimalistic with only `[...]` and `for ... of`. But don't worry. There are several ways to make those functional methods back again.
For instance, You can use [js-xiterable] like so:
[js-xiterable]: https://github.com/dankogai/js-xiterable
```javascript
import {xiterable as $X} from
'https://cdn.jsdelivr.net/npm/[email protected]/xiterable.min.js';
import {Permutation} from 'combinatorics.js';
let it = new Permutation('abcd');
let words = $X(it).map(v=>v.join(''))
for (const word of words)) console.log(word)
/*
abcd
abdc
...
dcab
dcba
*/
```