HTML / JS: Uncaught SyntaxError: Unexpected token ‘.’ when using an older browser [duplicate]

I have a script that retrieves an external JSON. It then checks if a sub-array is empty, and if it is, it does something and moves on. If the sub-array is not empty, it will check starting from 0 each element in the array until it detects that it has finished. Then, it will do something and continue. This is the code that I have:

                var alertCheckNumber = 0
                function checkAlert () {
                    
                    // if there are no alerts
                    if (data?.alerts===undefined) {
                        document.getElementById('allAlerts').style.display = "none";
                        return;
                    }
                    // proceed to check the number of alerts
                    if (data.alerts[alertCheckNumber]?.event===undefined) {
                        finalAlertNumber = alertCheckNumber;
                        var row = [ 0, 1, 2, 3, 4, 5, 6];
                        for (var i = 0; i<row.length; i++) {
                            if (row[i] >= finalAlertNumber) {
                                document.getElementById(`DIValertdescription${row[i]}`).style.display = "none";
                                document.getElementById(`DIValertlocation${row[i]}`).style.display = "none";
                                document.getElementById(`DIValertname${row[i]}`).style.display = "none";
                            }
                        }
                        return;
                    } else {
                        console.log(alertCheckNumber);
                        printAlertData(alertCheckNumber);
                        alertCheckNumber++
                        checkAlert();
                    }
                }

However, the issue in the code is where I have data?.alerts===undefined. This is supposed to check if data.alerts is empty. This works fine in modern browsers, but when I run it in an older browser, like Chrome in Windows 7, I get this error:

Uncaught SyntaxError: Unexpected token '.'

pointing to the line of code I highlighted above.
How might I be able to fix this, so that it is backwards compatible?