How to load JS file in another JS file in Node

Cannot load JS file in my app (getting undefined) and I want to emulate the same effect as the tag in the plain HTML.

I have tried

import Api from './api' -> tells me that none of the defined function is a function (don't have any circular dependencies), so my best guess it that Api was not initalized or something?

Tried module.exports on Api -> tells me that Api is undefined

Tried exports.Api -> tells me that the function which i try to call from the Api is not a function

I tried to require and a few more things, which I cannot even recall, and none of it seems to be working. Main issue is that I don't recognize the format of the JS file in question since I never seen a variable declared as a function that contains other functions, so explanation on that might come in handy tbh.

var Api = (function() {
  var requestPayload;
  var responsePayload;
  var messageEndpoint = '/api/message';

  var sessionEndpoint = '/api/session';

  var sessionId = null;

  // Publicly accessible methods defined
  return {
    sendRequest: sendRequest,
    getSessionId: getSessionId,

    // The request/response getters/setters are defined here to prevent internal methods
    // from calling the methods without any of the callbacks that are added elsewhere.
    getRequestPayload: function() {
      return requestPayload;
    },
    setRequestPayload: function(newPayloadStr) {
        requestPayload = JSON.parse(newPayloadStr);
    },
    getResponsePayload: function() {
      return responsePayload;
    },
    setResponsePayload: function(newPayloadStr) {
      responsePayload = JSON.parse(newPayloadStr);
    },
    setErrorPayload: function() {
    }
  };

  function getSessionId(callback) {
    var http = new XMLHttpRequest();
    http.open('GET', sessionEndpoint, true);
    http.setRequestHeader('Content-type', 'application/json');
    http.onreadystatechange = function () {
      if (http.readyState === XMLHttpRequest.DONE) {
        var res = JSON.parse(http.responseText);
        sessionId = res.session_id;
        callback();
      }
    };
    http.send();
  }

  // Send a message request to the server
  function sendRequest(text, context) {
    // Build request payload
    var payloadToWatson = {
      session_id: sessionId
    };

    payloadToWatson.input = {
      message_type: 'text',
      text: text,
    };

    if (context) {
      payloadToWatson.context = context;
    }

    // Built http request
    var http = new XMLHttpRequest();
    http.open('POST', messageEndpoint, true);
    http.setRequestHeader('Content-type', 'application/json');
    http.onreadystatechange = function() {
      if (http.readyState === XMLHttpRequest.DONE && http.status === 200 && http.responseText) {
        Api.setResponsePayload(http.responseText);
      } else if (http.readyState === XMLHttpRequest.DONE && http.status !== 200) {
        Api.setErrorPayload({
          'output': {
            'generic': [
              {
                'response_type': 'text',
                'text': 'Something went wrong.'
              }
            ],
          }
        });
      }
    };

    var params = JSON.stringify(payloadToWatson);
    // Stored in variable (publicly visible through Api.getRequestPayload)
    // to be used throughout the application
    if (Object.getOwnPropertyNames(payloadToWatson).length !== 0) {
      Api.setRequestPayload(params);
    }

    http.send(params);
  }
}());

Code above is provided by IBM (for the Watson Assistant I am trying to work with) and the code is for the Node.JS application which works fine. It works fine since the code above is simply included in the app through the tag in their index.html and voila, it works, but I don't have that ability (read below). My issue is that their app is also a client app and I want to transfer all of that 'back-end' stuff to my REST API and that is why I am trying to use the code above.