function user(f){
var b = document.getElementById(f).name;
var txtFile = new XMLHttpRequest();
var a = new Array();
var env = true;
txtFile.open("GET", "includes/usrlist.txt", true);
txtFile.onreadystatechange = function() {
  if (txtFile.readyState === 4) {  // Makes sure the document is ready to parse.
    if (txtFile.status === 200) {  // Makes sure it's found the file.
      allText = txtFile.responseText; 
      var usernames =  txtFile.responseText.split(',');
      for (i=0;i<usernames.length;i++){
       a.push(usernames[i]);
      }
        if (a.length != 0 && document.getElementById(f).value != ""){
        for(var i=0;i<a.length;i++){
if (document.getElementById(f).value == a[i]){
display_erreur('erreur', 'user', b);
env = false;
return false;
}
else if (document.getElementById(f).value != a[i]){
env = true;
return true;
}
}
}
else {
env = true;
return true;
}
    }
  }
}
txtFile.send(null);
return env;
}