# Box handler

Chain transformations on a value, then unwrap the result.

Canonical: https://1loc.dev/function/box-handler/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Usage lines executed in Node.js 22 and headless Chromium; no external services
Updated: 2026-10-28

## Implementation

```javascript
function boxHandler(value) { return { next: f => boxHandler(f(value)), done: f => f(value) }; }
```

## Usage

```javascript
const percent = text => boxHandler(text).next(t => t.replace('%', '')).next(Number.parseFloat).done(n => n * 0.01);
percent('50%'); // → 0.5
percent('12.5%'); // → 0.125
```

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

## Limits

- Each next returns a new box; nothing is mutated. An error thrown in a step propagates immediately.
- For steps that do not need the box, a pipe reads the same; see Compose functions from left to right.

The function does not validate its arguments. Add checks where it receives untrusted input.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/box-handler.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.