How can I create a dynamic file and then have user download it on a web app?

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FileSaver.js

5 19,960 2.6 JavaScript

An HTML5 saveAs() FileSaver implementation

If it's a relatively small amount of data (like 100mb or less), try https://github.com/eligrey/FileSaver.js it handles all the edge cases nicely.
StreamSaver.js

2 3,287 1.7 JavaScript

StreamSaver writes stream to the filesystem directly asynchronous

If it's larger, it's complicated... https://github.com/jimmywarting/StreamSaver.js generally works, but is not 100% reliable. https://github.com/jimmywarting/native-file-system-adapter is supposed to be like a newer version of it, but I struggled to get it working. You can also use https://web.dev/file-system-access/ but it's only supported in Chrome.
Appwrite

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native-file-system-adapter

0 269 3.8 JavaScript

File system, based on the spec reference implementation

If it's larger, it's complicated... https://github.com/jimmywarting/StreamSaver.js generally works, but is not 100% reliable. https://github.com/jimmywarting/native-file-system-adapter is supposed to be like a newer version of it, but I struggled to get it working. You can also use https://web.dev/file-system-access/ but it's only supported in Chrome.

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