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Asked: 2026-06-02T10:34:46+00:00

I’m looking for a robust way of creating a zip archive on the fly

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I’m looking for a robust way of creating a zip archive on the fly from information on a given page and making this available for download. Client-side zipping is a must since my script runs from a bookmarklet.

My first approach while I was more concerned with writing the rest of the script was just to post the information to a few lines of PHP running on my local server which zipped it and sent it back. This is obviously not suitable for a bookmarklet worth sharing.

I found JSZip earlier today, and I thought that’d be the end of it. This library works great when it works; unfortunately, the archives I’m creating frequently exceed a couple of MBs, and this breaks JSZip. (Note: I’ve only tested this on Chrome.)

Pure JS downloads also have the limitation of funky names due the data URI, which I intended to solve using JSZip’s recommended method, using Downloadify, which uses Flash. This made me wonder whether the size limitations on JS zip generating could be / have been overcome by using a similar interplay of Flash & JS.

I Googled this, but having no experience with Actionscript I couldn’t figure out quickly whether what I’m asking is possible. Is it possible to use a Flash object from JS to create relatively large (into the 10s of MBs) zip file on the client-side?

Thanks!

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1 Answer

  1. Editorial Team

    First of all some numbers:

    • Flash promises that uploads will work if the file is smaller than 100 Mb (I don’t know whether it means base 10 or base 16).

    • There are two popular libraries in Flash for creating ZIP archives, but read on first.

    • ZIP archiver is a program that both compresses and archives the data, and it does it in exactly this order. I.e. it compresses each file separately and then appends it to the entire archive. This yields worse compression rate but allows for iterative creation of the archive. With the benefit being that you can even start sending the archive before it is entirely compressed.

    • An alternative to ZIP is first to use a dedicated archiver and then to compress the entire archive at once. This, some times can achieve few times better compression, but the cost is that you have to process the data at once.

    • But Flash ByteArray.compress() method offers you native implementation of deflate algorithm, which is mostly the same thing you would use in ZIP archiver. So, if you had implemented something like tar, you could significantly reduce the size of the files being sent.

    • But Flash is a single-thread environment, so, you would have to be careful about the size of the data you compress, and, probably, will have to find it out empirically. Or just use ZIP – more redundancy, but easier to implement.

    I’ve used this library before: nochump. Didn’t have any problems. Although, it is somewhat old, and it might make sense to try to port it to use Alchemy opcodes (which are used for fast memory access significantly reducing the cost of low-level binary arithmentic operations such as binary or, binary and etc.) This library implements CRC32 algorithm, which is an essential part of ZIP archive and it uses Alchemy – so it should be considerably faster, but you would have to implement the rest on your own.

    Yet another option you might consider is Goole’s NaCl – there you would be able to choose from archiver and compression implementations because it essentially runs the native code, so you could even use bz2 and other modern stuff – unfortunately, only in Chrome (and users must enable it) or Firefox (need plugin).

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