Google-apps-script function CHAR() and CODE()

I am beginner in script programming and I would like to use the equivalent function CHAR() and CODE() in order to increment Alphabet characters according to an IF condition.

Example of formula I use in Google Sheets in the cells ("D11") "=IF(C11=C10;CHAR(CODE(A10)+1);"A")" .

I have written the following script:

function onFormSubmit(e) {

//Déclaration des variables
var SheetResponse = SpreadsheetApp.getActiveSheet();
var DerniereLigne =  SpreadsheetApp.getActiveSheet().getLastRow();
var DateToday = Utilities.formatDate(new Date(), Session.getScriptTimeZone(), 'ddMMYY');

//Intégration dsu suffixe alphabétqiue pour l'ID
if (SheetResponse.getRange(DerniereLigne,2).getValue() <> SheetResponse.getRange(DerniereLigne-1,2).getValue()) {
          var Alpha = SheetResponse.getRange(DerniereLigne,15).setValue("A");
        }
     else {
          Alpha = SheetResponse.getRange(DerniereLigne,15).setValue(CHAR(CODE(Alpha)+1);
        }

//Création de l'ID dans la derniére ligne et colonne "N"
SheetResponse.getRange(DerniereLigne,14).setValue(DateToday + Alpha);

}

Please, Could someone help me. Thank you in advance.