zstddec/README.md

ZSTD (Zstandard) decoder for Web and Node.js, using WebAssembly

# zstddec

[![Latest NPM release](https://img.shields.io/npm/v/zstddec.svg)](https://www.npmjs.com/package/zstddec)
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[ZSTD (Zstandard)](https://github.com/facebook/zstd) decoder for Web and Node.js, using WebAssembly.

## Installation

```shell
npm install --save zstddec
```

## API

```javascript
import { ZSTDDecoder } from 'zstddec';

const decoder = new ZSTDDecoder();

await decoder.init();

const decompressedArray = decoder.decode( compressedArray, uncompressedSize );
```

> **Limitations:** The decoder may fail with the error `wasm function signature contains illegal type` when the `uncompressedSize` is not known in advance and given to the `decode()` method. This is presumably a bug in the WASM bindings, which I am not yet sure how to fix.

## Contributing

To build the project locally, run:

```
yarn install
yarn build
```

To test changes:

```
yarn test
```

## Building from source

Compiled from https://github.com/facebook/zstd/tree/dev/build/single_file_libs, with the
following steps:

```shell
# build zstd
make
cd build/single_file_libs
./create_single_file_decoder.sh

# build wasm
emcc zstddeclib.c -s EXPORTED_FUNCTIONS="['_ZSTD_decompress', '_ZSTD_findDecompressedSize', '_ZSTD_isError', '_malloc', '_free']" -Wl,--no-entry -s WASM=1 -Oz -g0 -flto -s ALLOW_MEMORY_GROWTH=1 -s FILESYSTEM=0 -s STANDALONE_WASM=1 -DNDEBUG=1 -s PURE_WASI=0 -o zstddec.wasm

# build streaming wasm
emcc zstddeclib.c -s EXPORTED_FUNCTIONS="['_ZSTD_decompress', '_ZSTD_findDecompressedSize', '_ZSTD_createDCtx', '_ZSTD_decompressStream', '_ZSTD_freeDCtx', '_ZSTD_DStreamInSize', '_ZSTD_DStreamOutSize', '_malloc', '_free']" -Wl,--no-entry -s WASM=1 -Oz -g0 -flto -s ALLOW_MEMORY_GROWTH=1 -s FILESYSTEM=0 -s STANDALONE_WASM=1 -DNDEBUG=1 -s PURE_WASI=0 -o zstddec-stream.wasm

# encode WASM to base64
base64 -i zstddec.wasm > zstddec.txt
base64 -i zstddec-stream.wasm > zstddec-stream.txt
```

The base64 string written to `zstddec.txt` is embedded as the `wasm` variable at the bottom
of the source file. The rest of the file is written by hand, in order to avoid an additional JS
wrapper generated by Emscripten.

_Last built June 2025, zstd v1.5.7 (`f8745da6`), emscc v4.0.10_.

## License

JavaScript wrapper is provided under the MIT License, and the WASM ZSTD decoder is provided by Facebook under the BSD 3-Clause License.