Delete all Sheets except one, loop within a loop - google-apps-script

Question

I have a spreadsheet that contains a list of URLS for spreadsheets and the sheet name that I want to keep.
The idea is that I delete all sheets within each of those spreadsheets except the sheet I want to keep.
I have written a script but I can not seem to get it to work, I get an error message of "invalid argument line 20" but I can not figure out why.
I have pasted my code below, any help would be appreciated.
function DeleteImportedData(){
var s3 = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("URL's")
var s1 = SpreadsheetApp.getActiveSpreadsheet()
var sh1 = s1.getSheetByName("MasterSheet")
var copy = sh1.getRange(1,1,sh1.getLastRow(),sh1.getLastColumn()).getValues()
var getHeader = sh1.getRange(1,1,1,25).getValues()
for (var j = 1;j<s3.getLastRow(); j++){
var filterName = s3.getRange(j+1, 1).getValue()
var id = s3.getRange(j+1,2).getValue()
var open = SpreadsheetApp.openById(id)
var sheet = open.getSheets()[0];
var sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();
var sh3 = open.getSheetByName("STSOR - "+filterName)
}
for (i = 0; i <= sheets.length; i++) {
switch(sheets[i]) {
case sheet.getSheetName() === sh3:
break;
default:
open.setActiveSheet(sheet[i]);
open.deleteActiveSheet;
open.deleteSheet(del)
}
}
}

Answer 1

when you delete the first sheet from the list Sheet1, Sheet2, Sheet3 - Sheet2 changes its index to Sheet1. So it is better to delete sheets from the end
try to use the reverse loop
for (i = sheets.length-1; i >=0 ; i--){
... your code ...
}

Answer 2

There is something wrong between your variable sheet and sheets and your second loop.
Try this way :
for (var j = 1;j<s3.getLastRow(); j++){
// your code
// second loop inside the first loop
for (i = 0; i <= sheets.length; i++) {
switch(sheets[i]) {
case sheets[i].getSheetName() === sh3: //instead of 'sheet'
break;
default:
open.setActiveSheet(sheets[i]); //instead of 'sheet'
open.deleteActiveSheet;
open.deleteSheet(del)
}
} // close first loop
} // close second loop
but I think the second loop would be cleaner that way (IMHO):
for (i = 0; i <= sheets.length; i++) {
if (sheets[i].getSheetName() !== sh3) {
open.deleteSheet(sheets[i].getSheetName());
}
}
If this code doesn't answer your question, please provide more informations about your spreadsheets