MastodonCompressionStream JS API - Stefano Vazzoler

CompressionStream JS API

I recently came across the CompressionStream JavaScript API to allow me to seamlessly compress data on a browser.


I was working on a personal project where I had to upload a large CSV file to a server in Go for processing via a web UI. The challenge here was that the CSV file could be larger than 100MB which, given that I use the free tier of Cloudflare would not have been possible.

Initially, I considered some options such as requiring the CSV file to be compressed beforehand but that was a bad UX. But eventually I came across the CompressionStream API and the Stream API; the former having an 80+% adoption globally and being supported by all major browsers according to

Thanks to CompressionStream I am able to natively compress a file on the user's browser, upload it to the backend where in Go I could extract it and read the CSV entries on demand for processing and also send back a compressed reply to the user's browser which can be decompressed again and presented to the user as a downloadable file.

Code sample

The following JavaScript code demonstrates how to leverage CompressionStream to compress a file on the user's browser, upload it to a backend server, and handle the response for processing and downloading.

// Read a file picked by the user, compress it, and turn it into a Blob.
const file = document.getElementById("fileInput").files[0];
const compressStream = CompressionStream("gzip"));
const blob = await new Response(compressStream).blob();
// Upload data
const response = await fetch("/api/upload", {
    method: "POST",
    body: blob,
if (response.status != 200) {
    // TODO: Handle error
// Write the response to a Blob while decompressing it.
const decompressStream = response.body.pipeThrough(new DecompressionStream("gzip"))
const resultBlob = await new Response(decompressStream).blob();
// Provide a download link
document.getElementById("downloadLink").download = "file.csv"
document.getElementById("downloadLink").href = window.URL.createObjectURL(resultBlob);

Unfortunately, it's not yet possible to provide a ReadableStream to fetch as the request body (which would be more resource efficient compared to creating a Blob) as it is still an experimental API feature. You can find an example of this in Chrome's blog if you're interested to find out how it might work in the future.

On the server side I have the following Go code template that handles the API endpoint to stream the compressed CSV data and writes the response.

func handler(w http.ResponseWriter, r *http.Request) {
	// GZIP Reader
	gzipReader, err := gzip.NewReader(r.Body)
	if err != nil {
		log.Printf("failed to create gzipReader, err: %v", err)
	// CSV Reader
	csvReader := csv.NewReader(gzipReader)
	// Prepare to write response
	responseBuffer := bytes.Buffer{}
	gzipWriter, err := gzip.NewWriterLevel(&responseBuffer, gzip.BestCompression)
	if err != nil {
		log.Printf("failed to create gzip writer: %v", err)
	csvWriter := csv.NewWriter(gzipWriter)
	// Process entries
	for {
		// Read row
		record, err := csvReader.Read()
		if err == io.EOF {
		} else err != nil {
		// TODO: Process entry, optionally write to csvWriter
	// Close writers and submit response
	err = gzipReader.Close()
	if err != nil {
		log.Printf("error while closing gzipReader: %v", err)
	err = gzipWriter.Close()
	if err != nil {
		log.Printf("error while closing gzipWriter: %v", err)
	_, err = w.Write(responseBuffer.Bytes())
	if err != nil {
		log.Printf("error while writing response: %v", err)


This approach can be beneficial for enabling users to upload potentially large files from a website, especially when bandwidth or upload size are of concern.

I'm hoping that support for ReadableStream in fetch requests body improves soon, especially with Safari and Firefox.

Perhaps it's time for me to revisit my Rocket personal project, but that's for another blog post.