# xjb-as
> ECMAScript-compatible f64/f32 to string (dtoa/ftoa) for AssemblyScript, on xjb64-derived shortest-decimal cores
Latest version **0.1.0** (published 2026-06-09) · (MIT AND Apache-2.0) license · 0 weekly downloads
## Install
```sh
npm install xjb-as
pnpm add xjb-as
yarn add xjb-as
bun add xjb-as
```
## Health
**Score 65/100 (B)** — status: active.
Positive: has types; esm support; no vulnerabilities; high maintenance score; high quality score.
Warnings: low downloads; pre 1.0.
## Facts
| | |
|---|---|
| Version | 0.1.0 |
| Published | 2026-06-09 |
| First published | 2026-06-09 |
| Weekly downloads | 0 |
| License | (MIT AND Apache-2.0) |
| TypeScript types | bundled |
| Module format | ESM |
| Dependencies | 0 |
| Unpacked size | 88.4 KB |
| Known vulnerabilities | 0 |
| Install scripts | no |
| GitHub stars | 0 |
| Author | Jairus Tanaka |
| Maintainers | jairussw |
| Keywords | assemblyscript, dtoa, ftoa, float, double, xjb, schubfach, zmij, tostring, wasm, dragonbox |
## Links
- npm: https://www.npmjs.com/package/xjb-as
- Repository: https://github.com/JairusSW/xjb-as
- Homepage: https://github.com/JairusSW/xjb-as#readme
- Issues: https://github.com/JairusSW/xjb-as/issues
- npm.io page: https://npm.io/package/xjb-as
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## Recent versions
- 0.1.0 (latest) — 2026-06-09
## README
╦ ╦ ╦ ╔╗ ╔═╗ ╔═╗
╬ ║ ╠╩╗ ══ ╠═╣ ╚═╗
╝ ╚ ╚╝ ╚═╝ ╩ ╩ ╚═╝
The fastest ECMAScript-Number.toString-compatible
f64/f32 → string for
AssemblyScript, built on the
xjb / Żmij family of shortest-decimal cores.
Table of Contents
- [Installation](#installation)
- [Usage](#usage)
- [API](#api)
- [Performance](#performance)
- [Verification](#verification)
- [Benchmarks](#benchmarks)
- [Architecture](#architecture)
- [Credits](#credits)
- [License](#license)
- [Contact](#contact)
## Installation
```bash
npm install xjb-as
```
`xjb-as` is plain exported functions - no compiler transform required. The digit
kernel uses [SIMD](https://en.wikipedia.org/wiki/Single_instruction,_multiple_data) when available
and falls back to scalar [SWAR](https://en.wikipedia.org/wiki/SWAR) otherwise.
To enable SIMD, add to your `asc` command:
```bash
--enable simd
```
Or in your `asconfig.json`:
```json
{
"options": {
"enable": ["simd"]
}
}
```
## Usage
```typescript
import { dtoa, dtoa_buffered, ftoa, ftoa_buffered } from "xjb-as";
dtoa(3.14159); // "3.14159"
dtoa(1e21); // "1e+21"
dtoa(-0.0); // "0"
ftoa(0.1); // "0.1"
```
Writing a serializer with its own buffer? Skip the `String` allocation and write
UTF-16 straight into your buffer:
```typescript
// UTF-16 written directly into your buffer (>= 64 bytes); no allocation.
const codeUnits = dtoa_buffered(buffer, value);
```
`dtoa(x)` matches V8 `x.toString()` / `JSON.stringify(x)` byte-for-byte -
including `Infinity`, `-Infinity`, `NaN`, `-0 → "0"`, `1e21 → "1e+21"`,
`1e-7 → "1e-7"`, the fixed-vs-exponential thresholds, and the minimal-width
signed exponent per [ECMAScript Specification](https://tc39.es/ecma262/multipage/ecmascript-data-types-and-values.html#sec-numeric-types-number-tostring).
## API
```typescript
dtoa(value: f64): string // shortest round-trip, ECMAScript-formatted
dtoa_buffered(buffer, value: f64): u32 // write UTF-16 into buffer, return code-unit count
ftoa(value: f32): string // shortest round-trip, ECMAScript-formatted
ftoa_buffered(buffer, value: f32): u32 // write UTF-16 into buffer, return code-unit count
```
**Buffer contract.** `dtoa_buffered` / `ftoa_buffered` write the shortest decimal
as UTF-16 *directly* into `buffer` (no intermediate ASCII pass or widening);
`buffer` needs ≥ 64 bytes. The returned code-unit count is exact - the extra
headroom covers the in-register digit-block stores, which can overshoot the
logical end by up to one 8-char block.
## Performance
Charts are published per release to the `docs` branch via `npm run charts:publish`
(`scripts/publish-charts.sh`) and pinned here by version.
## Verification
`dtoa`/`ftoa` are checked against V8 ground truth (`Number::toString` for f64; the
exact BigInt shortest-round-trip for f32) - 0 failures across every power of two
(±1/±0xF ulp), every power of ten, tens of millions of random values, and a
generated assertion suite with 100% line/branch coverage.
```bash
# generate the as-test spec from the V8 oracle, then run the suite
npm run gen-spec
npm test
# build the verify wasm + check against V8 (fixed edge + 2M random; pass a count)
npm run verify
node scripts/dtoa/verify.mjs 3000
# open-ended differential fuzz vs V8 (seeded, repro-friendly; --runs/--time/--seed)
npm run fuzz
```
## Benchmarks
```bash
npm run bench -- --v8 --wavm
npm run charts:build -- --v8 --wavm
npm run charts:serve
```
## Binary size
Float→string code is mostly its pow10 lookup table. Build with `-Oz` (or
`-Os`) and the f64 table shrinks from ~9.9 KB to ~5 KB: a Dragonbox-style
**compressed anchor cache** reconstructed at load time (one multiply-by-5 per
in-between power), selected automatically via `ASC_SHRINK_LEVEL` and **bit-identical**
to the full table. The f32 path is a compact, self-contained core (~1.2 KB
hi-only table) — an f32-only build never pulls in the f64 machinery.
| build (`--runtime stub`, SIMD) | `-O3` | `-Oz` |
| ------------------------------ | ------: | ------: |
| `dtoa` (f64) | 13.4 KB | 9.0 KB |
| `ftoa` (f32) | 2.9 KB | 2.9 KB |
| both | 21.9 KB | 17.9 KB |
## Architecture
Three modules under `assembly/`:
- **`dtoa.ts`** (f64) — ports the upstream `xjb64` v2 shortest-decimal core.
- **`ftoa.ts`** (f32) — a self-contained port of xjb's compact
single-hi-multiply f32 core (24-bit significand ⇒ a hi-only table and a
narrow 64×24 product).
- **`xjb.ts`** — the shared f64 engine `dtoa.ts` builds on: 128-bit math, the
pow10 tables + loads (full, or the compressed anchor cache at `-Oz`), the
SIMD/SWAR digit kernel, and the UTF-16 layout writers.
The formatter lays out digits per the [ECMA-262 `Number::toString` decision tree](https://tc39.es/ecma262/multipage/ecmascript-data-types-and-values.html#sec-numeric-types-number-tostring) and stores UTF-16 directly.
## Credits
The shortest-decimal **digits** are identical to Ryū/Dragonbox (shortest,
round-to-nearest-even) - exactly what ECMA-262 mandates; only the surface
formatting differs.
## License
[MIT + Apache-2.0 notices](./LICENSE)
## Contact
Please send all issues to [GitHub Issues](https://github.com/JairusSW/xjb-as/issues) and to converse, please send me an email at [me@jairus.dev](mailto:me@jairus.dev)
- **Email:** Send me inquiries, questions, or requests at [me@jairus.dev](mailto:me@jairus.dev)
- **GitHub:** Visit the official GitHub repository [Here](https://github.com/JairusSW/xjb-as)
- **Website:** Visit my official website at [jairus.dev](https://jairus.dev/)
- **Discord:** Contact me at [My Discord](https://discord.com/users/600700584038760448) or on the [AssemblyScript Discord Server](https://discord.gg/assemblyscript/)
---
_Source: https://npm.io/package/xjb-as · Machine-readable twin of the npm.io package page. Health data is recomputed on every publish._