Git one repo for the whole project (frontend + node.js)

Let say I have a project the consists of following parts:

  1. Front-end App (JavaScript)
  2. Back-end App (Node.js)

Is it sensible to have one folder for the front-end app and node.js backend app? Or in what case it is reasonable to separate it into different repos?

Is the backend intrinsically tied to the front end, could you conceivably disconnect the back end and use it for another application? If you can use it with another application then it should be separate repositories, but if it cannot, then it would be better to keep it as two folders in the same repository.

I would use a main repo with two git submodules. One submodule for the front-end, and another for the back end.

Whenever there is release of compatible versions of both front-end and backend, you can update update the references on the main module and commit.