How to compress an existing, uncompressed zip file

[jakeboone02]

I'm using the AG Grid library in a project for work. The Excel export feature generates a .xlsx file, which is really just a .zip file with a different extension. The problem is that the generated file from AG Grid is not compressed, so exporting a significant amount of data can result in a large file.

(Opening the file in Excel and saving it will compress it, but I'd like the users to receive an already-compressed version.)

Currently I'm working around the issue by reading the contents of the generated file and copying everything into a new zip file with zip.js. The new file is compressed by default so it ends up being much smaller than the original.

What I want to know is if I can use zip.js to compress the initial file in place instead of copying its contents to a new file.

[gildas-lormeau]

If you want to implement this feature easily in 2 lines of code, you could use the import/export methods of the fs.FS API in zip.js.

Here is below an example of code, it reads an uncompressed zip file and creates a compressed zip file from it.

import * as zip from "https://unpkg.com/@zip.js/zip.js/index.js";

const uncompressedZipData = new Blob([new Uint8Array([
	0x50, 0x4B, 0x03, 0x04, 0x0A, 0x00, 0x00, 0x00, 0x00, 0x00, 0x5C, 0xBA,
	0x45, 0x40, 0xC8, 0x15, 0x94, 0x86, 0x8A, 0x04, 0x00, 0x00, 0x8A, 0x04,
	0x00, 0x00, 0x09, 0x00, 0x00, 0x00, 0x6C, 0x6F, 0x72, 0x65, 0x6D, 0x2E,
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])], { type: "application/zip" });

const zipFs = new zip.fs.FS();
await zipFs.importBlob(uncompressedZipData);
const compressedZipData = await zipFs.exportBlob();
console.log(compressedZipData);

You can run the code here: https://plnkr.co/edit/jqye59hgw6i8eMlB

However, from a technical point of view, this code does the same thing as yours, i.e. creating a new zip file. It does not update the original zip file, and this feature is not possible in zip.js.

[jakeboone02]

Thanks so much! It may not be much different behind the scenes, but the fs.FS-based code is much simpler so I'll probably use that.

[gildas-lormeau]

@jakeboone02 I noticed recently the export methods in the FS API did not use all the CPU cores. You can fix this issue by updating zip.js to the latest version. Otherwise, pass the option bufferedWrite: true when calling export methods (e.g. exportBlob({ bufferedWrite: true })). This should significantly improve performance on machines with a multi-core CPU.

[jakeboone02]

Cool, thanks!