how i can add a "PreSaveItem()" function inside the modern UI

I have a custom list inside our sharepoint on-premises 2013, where i added the following "PreSaveItem()" script inside the Edit/Create list forms:-

function PreSaveItem(){
//codes goes here
if( status == "Closed")
{

if( ana.trim() == null
|| ana.trim().length <= 1){
errormessage = number+") Please Enter 'Analysis' field before closing the item \n";
result = false;
number++;
}
if(category == "Complaint" && (conclusion.trim() == null || conclusion.trim().length <= 1)){
errormessage = errormessage+number+") Please Enter 'Conclusion' field before closing the item \n";
result = false;
number++;
}
}
if(errormessage.length >= 1)
{
alert(errormessage);

}
return result;
}

now i am planning to migrate the list to sharepoint online, and per my test the above script will work if i render the online list inside classic UI, while if i render the online list inside the modern UI, the script will not have any effect, so my question is about the approaches i can follow to have a function which mimic the "PreSaveItem()" function inside our modern online UI?

Thanks