I'm trying to set tests for my APIs collection. The test should call three times, with three different parameters for each request in the collection.

I have added a pre-request script to my collection, containing two requests:

let countryIso3List = pm.collectionVariables.get("CountryIso3List");

if(!countryIso3List || countryIso3List.length == 0) {
    countryIso3List = ["AFG", "SYR", "IDN"];
}

let countryIso3 = countryIso3List.shift();

pm.collectionVariables.set("CountryIso3", countryIso3);
pm.collectionVariables.set("CountryIso3List", countryIso3List);

Then in the test of the requests I have the following code:

const currentCountryIso3List = pm.collectionVariables.get("CountryIso3List");
if (currentCountryIso3List && currentCountryIso3List.length > 0){
    postman.setNextRequest(request.name);
} else {
    postman.setNextRequest(null);
}

pm.test("Status code is 200", function () {
    pm.response.to.have.status(200);
});

It works fine only for the first call, putting "AFG" in the variable CountryIso3 which is then used to make the request.

After this initial run, I see that the CountryIso3List contains "SYR,IDN" as string and it fails to make the shift() command and it stops.

I've tried modifying the Pre-Request script by adding a split on the collection variable:

let countryIso3List = pm.collectionVariables.get("CountryIso3List");

if(!countryIso3List || countryIso3List.length == 0) {
    countryIso3List = ["AFG", "SYR", "IDN"];
}
else{
    countryIso3List = pm.collectionVariables.get("CountryIso3List").split(',');
}


let countryIso3 = countryIso3List.shift();
pm.collectionVariables.set("CountryIso3", countryIso3);
pm.collectionVariables.set("CountryIso3List", countryIso3List);

but it works fine on the first cycle, then if I repeat the test it brokes saying:

TypeError: pm.collectionVariables.get(...).split is not a function

How can I fix it?