How can I get a data from get request from pure js?

I have a simple express app, and I am trying to listen to app.get() router from JS file but I am getting Error here:

    fetch('/').then(function(response) {
        return response.json();
    }).then(function(data) {
      console.log(data);
    }).catch(function() {
      console.log("Error");
    });

app.js

app.get('/',(req,res)){
// some data fetched from api
res.send(data)
}

How can i use fetch to get results from get request from pure JS file? TIA!