Can't get PKCE access token uses javascript fetch request

according to the de tools all parameters are sent
could it be that one of them is simply of wrong type?
if so I suppose it will be the calculation of the code_verifier
for such a small task i did not want to import a library so i have the following code:

var code_verifier = '';
var codeChallenge = '';  

  /**
   *
   */
  setDropboxCodes(){
    
    var codes = {};
    const base64Encode = (str) => {
      return str.toString('base64')
        .replace(/\+/g, '-')
        .replace(/\//g, '_')
        .replace(/=/g, '');
      }

    codeVerifier  = base64Encode(getRandomString(128));
    var code256 = _self.sha256(codes['codeVerifier'])
    .codeChallenge = base64Encode(code256);

  }

  /**
   *
   */
  getRandomString(length) {
    var randomChars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var result = '';
    for ( var i = 0; i < length; i++ ) {
        result += randomChars.charAt(Math.floor(Math.random() * randomChars.length));
    }
    return result;
  }