I want to decompress a GZIP string with JavaScript

user_2024913 0
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I have this GZIPed string: H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==

I created that with this website: http://www.txtwizard.net/compression

I have tried using pako to ungzip it.

import { ungzip } from 'pako';

const textEncoder = new TextEncoder();
const gzipedData = textEncoder.encode("H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==");
console.log('gzipeddata', gzipedData);
const ungzipedData = ungzip(gzipedData);
console.log('ungziped data', ungzipedData);

The issue is that Pako throws the error: incorrect header check

What am I missing here?

A JSbin

pako gzip javascript
2 Answers

This string is base64-encoded.

You first need to decode it to a buffer:

const gzippedString = 'H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==';
const gzippedBuffer = new Buffer(gzippedString, 'base64');

Then you can un(g)zip it:

const unzippedBuffer = ungzip(gzippedBuffer);

The result on ungzip is a Unit8Array. If you want to convert it back to a string you'll need to decode it again:

const unzippedString = new TextDecoder('utf8').decode(unzipped);
user_24227760

The "H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==" is a base64 encoded string, you first need to decode that into a buffer.

textEncoder.encode just encodes that base64 encoded string into a byte stream.

How to do that depend on whether you are in a browser or on nodejs.

node.js version

To convert the unzipped data to a string you further have use new TextDecoder().decode()

For node you will use Buffer.from(string, 'base64') to decode the base64 encoded string:

import { ungzip } from 'pako';

// decode the base64 encoded data
const gzipedData = Buffer.from("H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==", "base64");

console.log('gzipeddata', gzipedData);
const ungzipedData = ungzip(gzipedData);


console.log('ungziped data', new TextDecoder().decode(ungzipedData));

browser version

In the browser, you have to use atob, and you need to convert the decoded data to an Uint8Array using e.g. Uint8Array.from.

The conversion I used was taken from Convert base64 string to ArrayBuffer, you might need to verify if that really works in all cases.

// decode the base64 encoded data
const gezipedData = atob("H4sIAAAAAAAA//NIzcnJVyguSUzOzi9LLUrLyS/XUSjJSMzLLlZIyy9SSMwpT6wsVshIzSnIzEtXBACs78K6LwAAAA==")
const gzipedDataArray = Uint8Array.from(gezipedData, c => c.charCodeAt(0))

console.log('gzipeddata', gzipedDataArray);
const ungzipedData = pako.ungzip(gzipedDataArray);


console.log('ungziped data', new TextDecoder().decode(ungzipedData));
<script src="https://cdnjs.cloudflare.com/ajax/libs/pako/2.0.4/pako.min.js"></script>

user_19604550
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