Currently there is no any API to access MyMaps programmatically.

I can see a feature request for this in the public issue tracker:

https://issuetracker.google.com/issues/35820262

It looks like Google is evaluating the feasibility to implement the API, however, no timeline provided at the moment.

Please star this feature request to express your interest and receive further updates.

UPDATE

As of April 2018 it looks like Google decided do not implement the API for Google MyMaps and marked the aforementioned feature request as Infeasible.

I will try to address your question

1. Google MyMaps and Google Maps JavaScript API are different products managed by different teams at Google. I'm not aware about any third party library or tool that allows to use MyMaps functionality via Google Maps JavaScript API.

2. MyMaps tools are not related to drawing library of Google Maps JavaScript API as I mentioned before.

3. I can see the following feature request in the public issue tracker regarding the ability to access Google MyMaps via Google Maps JavaScript API. However, it doesn't look like the API team has set a high priority on this task.

TL;DR

Use Function.prototype as a noop callback to quickly get rid of the error message.

https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=Function.prototype

Full Explanation

According to the Google Maps API documentation, the callback parameter has been required for a very long time. In practice, however, Google has only recently (within the past few days) begun enforcing this requirement.

While it doesn't seem to break anything, it now shows an ugly error message in the console...

Loading the Google Maps JavaScript API without a callback is not supported.

How can I fix it?

The short answer is to specify a callback value. Set the name of the JavaScript function that you would like to trigger once the external API library has finished loading.

https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=FUNCTION_NAME

Be warned, you must specify a real function name! Otherwise you'll trigger an "is not a function" error message, and simply be trading one error message for another.

But I don't have/need a callback function!

If you don't actually have a callback function to run immediately after the library loads, you can use Function.prototype as a noop stand-in.

https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=Function.prototype

The Function.prototype method is native to JavaScript, and does absolutely nothing... It's exactly what you need in a situation like this!

For more information about Function.prototype, see this unrelated SO thread...

Maps Javascript API is a CLIENT-SIDE Service

The reason why you were having a hard time running the Maps Javascript on the server-side using node is because it was not made to be used that way. Although the Maps Javascript API includes Directions Service, it does not mean that you can use it server-side.

Please do note that it was called client-side service because it should be used with a visualization of the data. In this case, loading a map in the browser.

So what should you use then because you want to run the Directions API on the server-side?

Use the Standalone Directions API Web Service

documentation specifically says this:

The Directions API is a web service that uses an HTTP request to return JSON or XML-formatted directions between locations. Directions is available in several forms:

• as a standalone API
• as part of the client-side Maps JavaScript API
• for server-side use as part of the Client Libraries for Google Maps Web Services

To do this with node, you can use axios by requiring it on your javascript code. There's a code sample from the Directions API web service documentation but I made you a modified one to suit your needs. Especially in fetching steps, legs and overview_polyline.

Here's a code sample using the sample request you provided on your question:

var axios = require('axios');

var config = {
  method: 'get',
  url: 'https://maps.googleapis.com/maps/api/directions/json?origin=Geneva&destination=Basel&mode=%20transit&key=API_KEY',
  headers: { }
};

axios(config)
.then(function (response) {
  // This is the JSON object result for the directions request
  const result = response.data;
  // Logging the `legs` array
  console.log(result.routes[0].legs);
  // Logging the `steps` array
  console.log(result.routes[0].legs[0].steps);
  // Logging the `overview_polyline`
  console.log(result.routes[0].overview_polyline)
})
.catch(function (error) {
  console.log(error);
}); 

You just have to run this code on your terminal after setting up the node environment:

node index.js

And this will log the results you wanted on the console.

p.s. you can also use the NodeJs client library the MrUpsidown provided on the comment of your question: https://github.com/googlemaps/google-maps-services-js

Hope this helps!