app.get('/users', function(req, res) {
 // Your function to be called goes here.
});

$.ajax({
 type: 'GET'
 url: 'http://localhost:8000/users',
 success: function(response) {
  console.log(response);
 },
 error: function(xhr, status, err) {
   console.log(xhr.responseText);
 }
});

$("button").click(function(){
  $.ajax({url: "demo_test.txt", success: function(result){
    // your response
  }});
});

axios.post('api/tobackend/function',{user:'a',password:'b'})
     .then((res)=>console.log('response from server'))

router.post('api/tobackend/function',(reqs,resp)=>{console.log('backend')})