XMLHTTPRequest Error: Synchronous XMLHttpRequest o

I'm using an XMLHttpRequest to access SoundClouds Top Trending Tracks (https://api-v2.soundcloud.com/charts?kind=trending&genre=soundcloud:genres:all-music&client_id=1dff55bf515582dc759594dac5ba46e9&q=). I can't even get to parsing and all the fun stuff with it, because my console logs this error:

Synchronous XMLHttpRequest on the main thread is deprecated because of 
its detrimental effects to the end user's experience. For more help 
http://xhr.spec.whatwg.org/

I spend some time looking for an answer and - as far as I understand - the problem is some javascript within an request. So I looked into the SoundCloud API and found this little bit in every track's properties:

... "waveform_url":"https://wis.sndcdn.com/2AVcYzUmENai_m.json" ...

So all solutions I've found to similar problems wouldn't work, as I can't change how their API works. Maybe I'm doing something completely wrong and you guys can help.

Here is the complete function which causes the problem (I think):

function initialSearch() {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', "https://api-v2.soundcloud.com/charts?kind=trending&genre=soundcloud:genres:all-music&client_id=1dff55bf515582dc759594dac5ba46e9&q=", false);
    xhr.addEventListener("load", function() {
        initialArray = JSON.parse(xhr.response);
        }, false);
}

I call it on "DOMContentLoaded" if it changes anything.

Maybe you can help me. Thanks.

You are using XMLHttpRequest wrong. Do asynchronous request instead of synchronous. Here is a fixed version:

function initialSearch() {
    var xhr = new XMLHttpRequest();
    xhr.addEventListener("load", function() {
        initialArray = JSON.parse(xhr.response);
    }, false);
    xhr.open('GET', "https://api-v2.soundcloud.com/charts?kind=trending&genre=soundcloud:genres:all-music&client_id=1dff55bf515582dc759594dac5ba46e9&q=");
    xhr.send();
}

But even then, you will get No 'Access-Control-Allow-Origin' header is present on the requested resource error. Which is CORS browser policy error. You can only make requests to the same origin resources.

So if you can modify your server code, do that request from your server and change your client AJAX request to ask data from your server.