By “use JavaScript” for the front end, do you mean using HTML/JS as a UI, and then having C# code running that listens for UI events? If so, yes this should be possible as C# can handle NUI callbacks which the JS would trigger NUI callbacks - Cfx.re Docs

As for the use of Lua exclusively for the fxmanifest.lua, yes you can run a resource with no Lua code other than the manifest file.

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