My tool flatterer: https://lite.flatterer.dev/ converts deeply nested JSON to csv/xlsx, is done in web assembly in the browser.
It hard to prove that it is not sending data to a server, so it can be trusted. I know people could check dev tools but that is error prone and some users may not be able to do it.
I wish there was an easy way to prove this to users as it would make online tools like this much more attractive.
If you have a tech savvy audience they can also view your thing in an iframe with only sandbox="allow-scripts" to prove that it's not making network requests.
I wrote an html/js log viewer with those security models https://GitHub.com/ljw1004/seaoflogs - it handles up to 10kline log files decently, all locally.
Don't know how reliable this is though or whether a web developer could work around this.
Unless you were also using incognito or throwaway tab containers to discard stored data.