authentication trought javascript

Hi, I have done this using Google Apps Script, which is basically Javascript.
I am not a developer, so I am sure there are better/different ways, but it works, and maybe it will point you in the right direction.
To get a valid refresh token I have to manually login once.

function pbiInitialAuth(){
var base = "https://login.windows.net/common/oauth2/authorize";
var rtype = "?response_type=code";
var clientID = "&client_id=" + pbiClientID();
var resource = "&resource=https://analysis.windows.net/powerbi/api&redirect_uri=http://localhost:13526/Redirect";
var clientSecret = "&client_secret=" + pbiClientSecret();
var url = base + rtype + clientID + resource + clientSecret;
Logger.log(url);
}

The url variable contains a URL that you can open in Firefox and enter your login details.
The URL you will be redirected to contains a parameter &code=... This is the refresh token.
Once you have that, you can automatically get new access tokens by using something like this:

function pbiAccessToken(){
  
  var code = pbiRefreshTokenValue();
  var clientID = pbiClientID();
  var clientSecret = pbiClientSecret();
  var url = "https://login.microsoftonline.com/common/oauth2/token";
  var redirect = "http://localhost:13526/Redirect";
  
  var payload =
   {
     "client_id" : clientID,
     "client_secret" : clientSecret,
     "code": code,
     "grant_type": "authorization_code",
     "redirect_uri": redirect
   };
  
  
  var headers = {
    'Content-Type': 'application/x-www-form-urlencoded'
  };
  
  
  var options = {
    'method': 'post',
    'headers': headers,
    'payload': payload
  };
  
  var response = UrlFetchApp.fetch(url, options);
  var data = JSON.parse(response);
  return data.access_token;
}

Hope that helps, it works for me.