Question 392441 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – With Web Streams, how can I distinguish if a readable stream is compressed (gzip) or not?

I have a client-side JS aapplication where I want to use Web Streams to process some input file from the user. The file could either be a text file (in which case I just pipe it through a TextDecoderStream and then do whatever I want with it) or a gzipped text file (in which case I need to pipe it through a DecompressionStream before it gets to the TextDecoderStream).

What’s the best way to handle this?

I could make the user manually click a checkbox for compressed or not, but that’s poor UI.

I could look at the filename of the uploaded file, but that may not always be accurate.

I guess ideally I’d write some DecompressionStreamIfNecessary transform stream that I could just pipe through and it would decompress the stream if necessary or do nothing otherwise. idk if that’s possible though.

What’s the best way to do this?

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    I came up with this which seems to work, but the API is kind of ugly, it would be nicer if this was just a TransformStream I could pipe through. I would accept an answer from someone who is able to do that!

    // Check first 2 bytes of stream for gzip header
const isStreamGzipped = async (stream: ReadableStream) => {
    const reader = stream.getReader();
    const { value } = await reader.read();
    reader.cancel();

    if (value !== undefined && value.length >= 2) {
        return value[0] === 0x1f && value[1] === 0x8b;
    }

    return false;
}

const decompressStreamIfNecessary = async (inputStream: ReadableStream) => {
    const [checkGzipStream, outputStream] = inputStream.tee();

    if (await isStreamGzipped(checkGzipStream)) {
        return outputStream.pipeThrough(
            new DecompressionStream("gzip")
        );
    }

    return outputStream;
};

(await decompressStreamIfNecessary(stream))
    .pipeThrough(new TextDecoderStream())
    .pipeTo(whatever);

  2. 0

    Simply try to decompress, and if that fails, process it as text. The decompressor will likely give up in the first byte or two, since a text file won’t have a gzip header.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search