what3words API authentication with XMLHttpRequest

what3words-xmlhttprequest.js

function loadConfig(callback) {   
  var xobj = new XMLHttpRequest();
  xobj.overrideMimeType("application/json");
	xobj.open('GET', './config.json', true);
	xobj.onreadystatechange = function () {
    if (xobj.readyState == 4 && xobj.status == "200") {
      // Required use of an anonymous callback as .open will NOT return a value but simply returns undefined in asynchronous mode
      callback(xobj.responseText);
    }
  };
  xobj.send(null);  
 }

function init() {
  loadConfig(function(response)) {
    json = JSON.parse(response);
  }
}

var json = {};
init();
var key = json.what3words.key;

var options = {
  key: key
  lang: 'en'
};
var w3w = new What3words(options);