How can I call a Node.js function from a client-side event listener function?

Question

Good afternoon!

I came across an interesting challenge (for a javascript newcomer like me).

Scenario:

I have a HTML select element with 3 option elements (experiment 1, experiment 2, experiment 3). If I click on experiment 1 I would like to call the getPatientDataFromExperiment() function from Node.js file dataHandler.js within my addEventListener function of the select element. The event listener works fine. But I don't get access to the function. During my research I came across topics which could potentially solve my problem, but to be honest - I didn't quite understand them (especially the socket solution). I am using Node.js (v8.9.4) and Visual Studio Code.

What have I done so far:

I tried stupidly to load the script which contains the function via a XmlHttpRequest. But in the corresponding response I've just got my script back as a string.

Question:

Is there a more or less simple solution to call a server-side function of a Node.js file from a client-side event listener function (especially without refreshing the site)?

I would be very grateful for your precious help and a demo example. Thank you very much!

Research:

node.js executing server side function from client side

How to call a server side function from client side (e.g. html button onclick) in Node.js?

Code (Client-side, eventHandler.js):

document.getElementById("experiment").addEventListener("change", function()
{
    var request = new XMLHttpRequest();
    request.onreadystatechange = function()
    {
        if (this.readyState == 4 && this.status == 200) 
        {
            document.getElementById("target3").innerHTML =
            this.responseText;
        }
    }

    request.open("GET","javascript/dataHandler.js",true);
    request.send();
});

Code (Node, dataHandler.js):

function getPatientDataFromExperiment()
{
    alert("Function call worked");
}

Result: getPatientDataFromExperiment function all as plain text

Answer 1

1) You need to host the dataHandler.js
2) Don't use alert in a NodeJS project
If I were you I would use ExpressJS. Create a new folder in somewhere but not inside your frontend project. Then run

npm init
npm install --save-dev express

Then create a file there called server.js with this content:

const express = require('express')
const app = express()

app.get('/', (req, res) => res.send('Function call worked'))

app.listen(3000, () => console.log('Example app listening on port 3000!'))

Then you can host it in your localhost with the following command:

node server.js

Then you need to modify this line in your frontend code

request.open("GET","javascript/dataHandler.js",true);

to this:

request.open("GET","http://localhost:3000",true);