How to POST to Spotify API token endpoint with JavaScript?

I'm running following js-code. But I keep getting an error from Spotify Webpage (saying there was some sort of error). credentials is a base64 encoded string of client_id and secret.

The curl-command works just fine:

curl -X "POST" -H "Authorization: Basic *credentials*" -d grant_type=client_credentials https://accounts.spotify.com/api/token

The js-code doesn't work properly..

I guess it's pretty easy, but I'm not that good in js (sorry!).

fetch('https://accounts.spotify.com/api/token', {
    method: 'POST',
    headers: {'Authorization': 'Basic *credentials*'},
    body: 'grant_type=client_credentials'
    })
    .then(response => response.json())
    .then(data => {
        console.log(data);
    });

As specified in the Spotify Authorization guide the body has to be application/x-www-form-urlencoded so just add this header:

fetch('https://accounts.spotify.com/api/token', {
    method: 'POST',
    headers: {
        'Authorization': 'Basic *credentials*',
        'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
    },
    body: 'grant_type=client_credentials'
})
.then(response => response.json())
.then(data => {
    console.log(data);
});