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0.1.1 • Public • Published


High performance Zstandard decompression in a pure JavaScript, 8kB package



// I will assume that you use the following for the rest of this guide
import * as fzstd from 'fzstd';

If your environment doesn't support ES Modules (e.g. Node.js):

const fzstd = require('fzstd');

If you want to load from a CDN in the browser:

You should use either UNPKG or jsDelivr (i.e. only one of the following)

You may also want to specify the version, e.g. with fzstd@0.01
<script src="https://unpkg.com/fzstd"></script>
<script src="https://cdn.jsdelivr.net/npm/fzstd/umd/index.js"></script>
<!-- Now, the global variable fzstd contains the library -->

<!-- If you're going buildless but want ESM, import from Skypack -->
<script type="module">
  import * as fzstd from 'https://cdn.skypack.dev/fzstd?min';

If you are using Deno:

// Don't use the ?dts Skypack flag; it isn't necessary for Deno support
// The @deno-types comment adds TypeScript typings

// @deno-types="https://cdn.skypack.dev/fzstd/lib/index.d.ts"
import * as fzstd from 'https://cdn.skypack.dev/fzstd?min';

And use:

// This is an ArrayBuffer of data
const compressedBuf = await fetch('/compressedData.zst').then(
  res => res.arrayBuffer()
// To use fzstd, you need a Uint8Array
const compressed = new Uint8Array(compressedBuf);
// Note that Node.js Buffers work just fine as well:
// const massiveFile = require('fs').readFileSync('aMassiveFile.txt');

const decompressed = fzstd.decompress(compressed);

// Second argument is optional: custom output buffer
const outBuf = new Uint8Array(100000);
// IMPORTANT: fzstd will assume the buffer is sufficiently sized, so it
// will yield corrupt data if the buffer is too small. It is highly
// recommended to only specify this if you know the maximum output size.
fzstd.decompress(compressed, outBuf);

You can also use data streams to minimize memory usage while decompressing.

let outChunks = [];
const stream = new fzstd.Decompress((chunk, isLast) => {
  // Add to list of output chunks
  // Log after all chunks decompressed
  if (isLast) {
    console.log('Output chunks:', outChunks);

// You can also attach the data handler separately if you don't want to
// do so in the constructor.
stream.ondata = (chunk, final) => { ... }

// Since this is synchronous, all errors will be thrown by stream.push()
// Last chunk must have the second parameter true
stream.push(chunkLast, true);

// Alternatively, you can push every data chunk normally and push an empty
// chunk at the end:
// stream.push(chunkLast);
// stream.push(new Uint8Array(0), true);


Unlike my Zlib implementation fflate, WebAssembly ports of Zstandard are usually significantly (40-50%) faster than fzstd. However, they fail to decompress most archives without the decompressed size provided in advance. Moreover, they do not support streaming and thereby allocate a large amount of memory that cannot be freed. Lastly, fzstd is absolutely tiny: at 8kB minified and 3.8kB after gzipping, it's much smaller than most WASM implementations.

Please note that unlike the reference implementation, fzstd only supports a maximum backreference distance of 225 bytes. If you need to decompress files with an "ultra" compression level (20 or greater) AND if your files can be above 32MB decompressed, fzstd might fail to decompress properly. Consider using a WebAssembly port for files this large (though this may be difficult if you don't know the decompressed size).

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