Google Apps Script; how to solve TypeError: Cannot read properties of undefined (reading '0')? - google-apps-script

Question

I have a Google sheet which I follow stock prices Daily; every day I get the stock prices online and write them to the sheet via apps script. Now I want to check monthly P/L for each stock and create a sheet that shows monthly P/L for each stock.
I am trying to get the prices with dates for each stock in an array and then push the first and last entries for every month in "monthly" array. And then I am trying to write this monthly values in "Aylık P/L" sheet.
The code works fine till I added "for" statement to write the monthly datas to the sheet; it throws: "Error in Monthly: TypeError: Cannot read properties of undefined (reading '0')":
Here goes the code:
function aylikRapor() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet1 = ss.getSheetByName('Fiyat Çek');
var sheet2 = ss.getSheetByName('Günlük Fon Takip');
var sheet3 = ss.getSheetByName('Haftalık Fon Takip');
var sheet4 = ss.getSheetByName('Aylık Fon Takip');
var sheet5 = ss.getSheetByName('Funds Main');
var sheet6 = ss.getSheetByName('Aylık');
var sheet7 = ss.getSheetByName('Aylık P/L');
var pfs = sheet5.getRange(1,1).getValue(); // pfs = PortföydekiFonSayısı
var i = 1;
var f = 1;
var a;
var b;
try {
for(i=1; i<=pfs; i++) {
var colH = sheet2.getRange(1,3*i,1,1);
var harf = colH.getA1Notation().match(/([A-Z]+)/)[0];
var column = sheet2.getRange(harf+":"+harf);
// get lastFilledRow
var value = ''
const max = sheet2.getMaxRows()
var values = column.getValues()
values = [].concat.apply([], values)
for (row = max - 1; row > 0; row--) {
value = values[row]
if (value != '') { break }
}
lastRowD = row + 1;
Logger.log(lastRowD);
let veriler = sheet2.getRange(14,3*i-1,lastRowD-12,2).getValues();
let month = veriler[0][0].getMonth();
let monthly = [[veriler[0][0],veriler[0][1]]];
veriler.forEach( (row,index) => {
let test = new Date(row[0]).getMonth();
if( test !== month ) {
monthly.push([veriler[index-1][0],veriler[index-1][1]]);
monthly.push([row[0],row[1]]);
month = test;
}
}
)
Logger.log(monthly);
for (f=1, a=0, b=1; f <=monthly.length-1; f++, a+=2, b+=2){
sheet7.getRange(3+f,5*i-2).setValue(monthly[a][0]);
sheet7.getRange(3+f,5*i-1).setValue(monthly[a][1]);
sheet7.getRange(3+f,5*i).setValue(monthly[b][0]);
sheet7.getRange(3+f,5*i+1).setValue(monthly[b][1]);
}
}
}
catch(err) {
Logger.log("Error in Monthly: "+err);
}
}
I think there is a null value in "monthly" array; but I could not figure out how I can solve it!
Execution Log:
Yürütme günlüğü
09:49:37 Bildirim Yürütme işlemi başlatıldı
09:49:49 Bilgi 119.0
09:49:50 Bilgi [[Wed Aug 17 00:00:00 GMT+03:00 2022, 0.080777], [Wed Aug 31 00:00:00 GMT+03:00 2022, 0.082585], [Thu Sep 01 00:00:00 GMT+03:00 2022, 0.0831], [Fri Sep 30 00:00:00 GMT+03:00 2022, 0.08431], [Mon Oct 03 00:00:00 GMT+03:00 2022, 0.085019], [Mon Oct 31 00:00:00 GMT+03:00 2022, 0.096867], [Tue Nov 01 00:00:00 GMT+03:00 2022, 0.098144], [Wed Nov 30 00:00:00 GMT+03:00 2022, 0.115665], [Thu Dec 01 00:00:00 GMT+03:00 2022, 0.117343], [Fri Dec 30 00:00:00 GMT+03:00 2022, 0.126554], [Mon Jan 02 00:00:00 GMT+03:00 2023, 0.127234], [Thu Jan 12 00:00:00 GMT+03:00 2023, 0.111894], [, ]]
09:49:50 Bilgi Error in Monthly: TypeError: Cannot read properties of undefined (reading '0')
09:49:38 Bildirim Yürütme işlemi tamamlandı

Answer 1

I have fixed the problem:
As you can see from the execution log; for i=1 the first monthly array length is 12; during the for (f=1, a=0, b=1; f <=monthly.length-1; f++, a+=2, b+=2)
the max f should be 6; so after 6 there would be no values in the array.
I fixed the problem by the following amendment:
(f=1, a=0, b=1; f <=monthly.length/2; f++, a+=2, b+=2)
I hope the others could get some help in case problems like this.