Store JSON response from a function into a variable - Error "undefined" is not valid JSON

Question

this is my second question today but I feel I'm so close to get this right that I just had to do it...

Long story short: I'm developing a flashcard web app using JSON files as a database. My previous question: Loop through JSON object properties and show one at a time in a text button every time is clicked - flashcard concept.

I managed to get the response from the JSON file using a function (getSSIIData). However, when I try to store the result in a variable (jsonSSII) so that I can manipulate it afterwards, I get this error:

"VM454:1 Uncaught SyntaxError: "undefined" is not valid JSON
    at JSON.parse (<anonymous>)
    at app.js:28:24"

Which is confusing, because when I try this with an array of objects stored in a variable in the same Javascript file it DOES work. E.g:

var jsonSSII = `[
  {
    "subject": "Database",
    "concept": "Relational data model",
    "definition": "An abstract model used to organize and manage the data..."
  },
  {
    "subject": "Database",
    "concept": "Information system",
    "definition": "An integrated set of components for collecting, storing..."
  }
]`;

However, it does not work with the text response I get from the JSON file, although it looks exactly the same when I console.log it.

This is what I have so far:

var clicks = -1;
var button = document.querySelector("#ssii-button");

var jsonSSII = (function getSSIIData() {
  const requestData = new XMLHttpRequest();

  requestData.open("GET", "data-ssii.json", true);

  requestData.send();

  requestData.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      data = this.responseText;
      console.log(this.responseText);
      return data;
    }
  };
})();

var contentSSII = JSON.parse(jsonSSII);
const buttonText = contentSSII.reduce((acc, cur) => {
  for (let key in cur) {
    if (key != "subject") {
      acc.push(cur[key]);
    }
  }

  return acc;
}, []);

function showContent() {
  clicks += 1;
  button.innerHTML = buttonText[clicks];
}
button.addEventListener("click", showContent);

Thanks a million times for your help!