How to check if a JSON file exists?

Question

JavaScript

Hello.
Task: if A.json does not exist, load B.json
Why won't my bike move? Thank you.
https://jsfiddle.net/kukaew/16dqjx5g/

function cursArr() {
  var	xhr = new XMLHttpRequest();
    xhr.open('GET', 'A.json', false);
    xhr.send();
  if (xhr.status != 200) {
    xhr.open('GET', 'B.json', false);
    xhr.send();
               return JSON.parse(xhr.responseText);
  }
  else {
    return JSON.parse(xhr.responseText);
  }
}