# 
[](https://www.npmjs.com/package/@thi.ng/bidir-index)
[](https://mastodon.thi.ng/@toxi)
> [!NOTE]
> This is one of 216 standalone projects. LLM-free, human-made and
> cared for software, maintained as part of the
> [@thi.ng/umbrella](https://codeberg.org/thi.ng/umbrella/) ecosystem and
> anti-framework.
>
> 🚀 Please help me to work full-time on these projects by [sponsoring
> me](https://codeberg.org/thi.ng/umbrella/src/branch/develop/CONTRIBUTING.md#donations).
> Thank you! ❤️
- [About](#about)
- [Status](#status)
- [Related packages](#related-packages)
- [Installation](#installation)
- [Dependencies](#dependencies)
- [API](#api)
- [Basic usage example](#basic-usage-example)
- [Authors](#authors)
- [License](#license)
## About
Bi-directional index mapping arbitrary keys to numeric IDs & vice versa.
This package contains functionality which was previously part of and has been
extracted from the [@thi.ng/associative](https://thi.ng/associative) package.
## Status
**STABLE** - used in production
[Search or submit any issues for this package](https://codeberg.org/thi.ng/umbrella/issues?q=%5Bbidir-index%5D)
## Related packages
- [@thi.ng/associative](https://codeberg.org/thi.ng/umbrella/src/branch/develop/packages/associative) - ES Map/Set-compatible implementations with customizable equality semantics & supporting operations
## Installation
```bash
yarn add @thi.ng/bidir-index
```
ESM import:
```ts
import * as bi from "@thi.ng/bidir-index";
```
Browser ESM import:
```html
```
[JSDelivr documentation](https://www.jsdelivr.com/)
For Node.js REPL:
```js
const bi = await import("@thi.ng/bidir-index");
```
Package sizes (brotli'd, pre-treeshake): ESM: 1.04 KB
## Dependencies
- [@thi.ng/api](https://codeberg.org/thi.ng/umbrella/src/branch/develop/packages/api)
- [@thi.ng/checks](https://codeberg.org/thi.ng/umbrella/src/branch/develop/packages/checks)
Note: @thi.ng/api is in _most_ cases a type-only import (not used at runtime)
## API
[Generated API docs](https://docs.thi.ng/umbrella/bidir-index/)
TODO
## Basic usage example
```ts tangle:export/readme.ts
import { defBidirIndex, encodeObject, decodeObject } from "@thi.ng/bidir-index";
const index = defBidirIndex();
// given object keys are auto-indexed, array initialized with given default
console.log(
encodeObject(index, { r: 1, g: 2, b: 3, a: 4 }, 0)
);
// [1, 2, 3, 4]
// use custom default and without updating index
console.log(
encodeObject(index, { b: 3, r: 1, g: 2 }, -1, false)
);
// [1, 2, 3, -1] (missing key `a` mapped to given default value)
// decode with defaults/fallback
console.log(
decodeObject(index, [255, 128, 64], { a: 1 })
);
// { r: 255, g: 128, b: 64, a: 1 }
// add more keys to index (already known ones will be skipped)
// returns array of mapped IDs for given keys
index.addAll(["r", "g", "b", "a", "foo"]);
// [0, 1, 2, 3, 4]
// decoding will skip nullish values
console.log(
decodeObject(index, [null, null, null, null, "bar"])
);
// { foo: "bar" }
```
## Authors
- [Karsten Schmidt](https://thi.ng)
If this project contributes to an academic publication, please cite it as:
```bibtex
@misc{thing-bidir-index,
title = "@thi.ng/bidir-index",
author = "Karsten Schmidt",
note = "https://thi.ng/bidir-index",
year = 2022
}
```
## License
© 2022 - 2026 Karsten Schmidt // Apache License 2.0