Encode and decode base64 GZIP with pure JavaScript. Works in all modern browsers.

gzip.js

// Paste the following example to browser console


// Comppreses string to GZIP. Retruns a Promise with Base64 string
const compress = string => {
    const blobToBase64 = blob => new Promise((resolve, _) => {
        const reader = new FileReader();
        reader.onloadend = () => resolve(reader.result.split(',')[1]);
        reader.readAsDataURL(blob);
    });
    const byteArray = new TextEncoder().encode(string);
    const cs = new CompressionStream('gzip');
    const writer = cs.writable.getWriter();
    writer.write(byteArray);
    writer.close();
    return new Response(cs.readable).blob().then(blobToBase64);
};

// Decompresses base64 encoded GZIP string. Retruns a string with original text.
const decompress = base64string => {
    const bytes = Uint8Array.from(atob(base64string), c => c.charCodeAt(0));
    const cs = new DecompressionStream('gzip');
    const writer = cs.writable.getWriter();
    writer.write(bytes);
    writer.close();
    return new Response(cs.readable).arrayBuffer().then(function (arrayBuffer) {
        return new TextDecoder().decode(arrayBuffer);
    });
}

/////////////////////
// Checking
/////////////////////
Promise.resolve("Hello GZIP!")
    .then(v => {
        console.log("Original value: %s", v);
        return v;
    })
    .then(v => compress(v))
    .then(v => {
        console.log("Compressed value: %s", v);
        return v;
    })
    .then(v => decompress(v))
    .then(v => console.log("Decomporessed value: %s", v));

Thanks @asidko I was trying to roll my own version of this but ran into an error from the Response: "TypeError: Failed to fetch". I believe it was something to do with my approach to base64 encoding. Your implementation worked a treat 👍