Preview:
/**
  * getAllAllowedFieldsForCurrentUser
  * @param objectAPI, Object Api Name
  * @return String.join(fieldList, ','), all the fields concatened, separated by ","
  */
private static string getAllAllowedFieldsForCurrentUser (string objectAPI) {
    return string.join(new List<String>(Schema.getGlobalDescribe().get(objectAPI).getDescribe().fields.getMap().keySet()), ',');
}
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