Given 3 parameters, I'd like to know why this is not iterating/generating the dates, but repeating the first iteration.

//Means 01 installement for tomorrow and the other 6 from tomorrow on every 30 days
function generateInstallments() {
let condition = '1 + 6x'
let firstInstallment = 1;//Tomorrow
let intervalInDays = 30;
var dueDate = '';
let deadlines = [];

if (condition.indexOf('+') > -1) { //If it contains this pattern (1 + 4x)
    //Extracts the number preceding x, as this will be the number of installments
    condition= Number(condition.slice(
      condition.indexOf('+ ') + 2,
      condition.lastIndexOf('x'),
    ));

    let firstInstallementDate = addDays(new Date(), firstInstallment );
    deadlines.push(Utilities.formatDate(firstInstallementDate , Session.getTimeZone(), "dd/MM/yyyy"));

    for (let a = 0; a < condition; a++) {
      dueDate = addDays(firstInstallementDate , intervalInDays );//Function can be found below
      deadlines.push(Utilities.formatDate(dueDate, Session.getTimeZone(), "dd/MM/yyyy"));
    }
  }
}

The first and 2 second dates are correct, but the other 4 are the same as the second one, so this is not iterating as expected.

This is the function I'm using to add the days:

function addDays(date, days) {
  var result = new Date(date);
  result.setDate(result.getDate() + days);
  return result;
}

Thank you!