Basic XHR example

HTML

<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title>Get coordinates of the mouse pointer</title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.52.0/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.52.0/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>
<input type="button" id="basicXHRButton" value="Try It">

<style type='text/css'>
    #info {
        display: block;
        position: relative;
        margin: 0px auto;
        width: 50%;
        padding: 10px;
        border: none;
        border-radius: 3px;
        font-size: 12px;
        text-align: center;
        color: #222;
        background: #fff;
    }
</style>
<div id='map'></div>
<pre id='info'></pre>
<script>
mapboxgl.accessToken = 'pk.eyJ1IjoidmljbmF1bSIsImEiOiJjanE5aDYzMHcwMTlyNDJtNTFqNGZrOHNuIn0.WbWM_PiCcIgTDCwgz0qF9w';
var map = new mapboxgl.Map({
    container: 'map', // container id
    style: 'mapbox://styles/mapbox/streets-v9',
    center: [18.54, 54.50], // starting position
    zoom: 11 // starting zoom
});

map.on('mousemove', function (e) {
    document.getElementById('info').innerHTML =
        // e.point is the x, y coordinates of the mousemove event relative
        // to the top-left corner of the map
        JSON.stringify(e.point) + '<br />' +
        // e.lngLat is the longitude, latitude geographical position of the event
        JSON.stringify(e.lngLat);
});

function pointOnCircle(lat, long) {
    return {
        "type": "Point",
        "coordinates": [lat, long]
    };
}

map.on('load', function () {
  map.addSource('point', {
        "type": "geojson",
        "data": pointOnCircle(0, 0)
  });

map.addLayer({
        "id": "point",
        "source": "point",
        "type": "circle",
       ...

JavaScript

document.getElementById("basicXHRButton").addEventListener("click", function() {
        // Loading the jQuery code
        request = new XMLHttpRequest();
        request.open("GET", "https://cors.io/?https://restaurant-api.wolt.com/v1/pages/front?lat=54.4714111&lon=18.4683329", true);
        request.send(null);

        request.onreadystatechange = function() {
            if(request.readyState === 4) { // What does this even mean?
                if(request.status === 200) {
                    alert(JSON.parse(request.responseText)["city"]);
                }
            }
        }
    });