NodeService: Regarding calling node.js function from server side function

i read one nice article from http://rion.io/2017/08/22/nodeservices-where-javascript-and-net-meet-back-on-the-other-side/

Rion show how to call node.js function from server side function see a example.

module.exports = function(a, b, callback) {
  let result = a + b;
  callback(result);
};

public async Task<long> Add(int x = 11, int y = 31)
{
    return await _nodeServices.InvokeAsync<long>("Scripts/Add.js", x, y);
}

this way we are calling node.js function from server side area. _nodeServices.InvokeAsync<long>("Scripts/Add.js", x, y);

my question is we are not mention function name why? suppose add.js has many function then how could we call any one of then.

i hope i could express my concern that how to call any function by name in node service. so please some one put some lights on it. thanks