but-unzip

but-unzip

small unzip library. ~704 bytes for Node, and ~944^ bytes for browsers, before gzip.

^92.5%+ of browsers support the decompression API, which in 2025, is probably all your users. If you really care about the last 7.5%, you can dynamically import pako, adding ~20k: see below.

Usage

Install via your favorite package manager and import but-unzip. Has zero dependencies.

$ npm install but-unzip

# for old browsers you need
$ npm install pako

This library returns zip entries synchronously, but only returns an entry's uncompressed bytes after calling .read(), which'll give Uint8Array or Promise<Uint8Array>.

Naïve use

If there's a built-in function to inflate compressed files (like in Node or most browsers), you can use the code like:

import { iter } from 'but-unzip';
import * as fs from 'fs';

const bytes = fs.readFileSync('somezip.zip');

for (const entry of iter(bytes)) {
  console.info(entry.name, entry.comment);
  const bytes = await entry.read();
  // do something with bytes
}

Provide inflate function

If you're worried about maximum compatibility:

import { unzip, inflateRaw as platformInflateRaw } from 'but-unzip';
import { inflateRaw as pakoInflateRaw } from 'pako/lib/inflate.js';

async function decompressUint8Array(zipBytes) {
  const allEntries = unzip(zipBytes, platformInflateRaw || pakoInflateRaw);
  // do something with entries
}

Dynamically import inflate

You could use dynamic import() instead to include pako, but nearly all users who are missing the compression library probably also don't support ESM imports (e.g., IE11 and old browsers).

So this is how you'd do it but I'd probably not recommend it:

import { inflateRaw as platformInflateRaw } from 'but-unzip';
const inflateRaw = platformInflateRaw || (await import('pako/lib/inflate.js').inflateRaw);

Limitations

• This library doesn't support ZIP64, but probably should. But your browser (and Node) will probably not be happy to work with 4gb+ files, especially as this is not a streaming library (it just gives everything at once).

• Like literally every zip library that exists, this only supports compression types 0 (store) and 8 (deflate).

Notes

• In my testing with esbuild, Pako's ESM bundling can be a bit broken, so importing "pako/lib/inflate.js" adds ~20k. Importing pako wholesale, even if you only use inflateRaw, adds ~45k.

• If you're handling user data and it could be really big, use a Worker. But also, the compression API is async and doesn't block your main thread. YMMV!