Is there any node package available for converting gltf to fbx?
I’m not aware of one. Can I ask why you want to do this? glTF is intended to be the last mile, end of the content pipeline, so normally people want to convert FBX to glTF and not the other way around.
I was trying to create something like a model viewer and in which the user can download the model in any format.
Is the model viewing engine expecting FBX then?
Typically you need to use the native FBX SDK to do much with FBX. It’s possible to call native code from node.js, but writing a full converter with the SDK is probably a fair amount of work.