Instead of forcing Date objects into their millisecond-since-The-Epoch values, it's better to directly obtain the milliseconds-since-The-Epoch value and utilize it in the subtraction operation.
You haven't provided information about what type a.date and b.date are, but we can assume they might be strings, numbers, or perhaps even instances of Date.
If a.date and b.date are strings
In the case where a.date and b.date are strings, you can leverage Date.parse to parse the strings with rules similar to those of new Date, allowing you to directly obtain the milliseconds-since-The-Epoch value:
return Date.parse(b.date) - Date.parse(a.date);
Please note that both this approach and the original code make an assumption that a.date and b.date are appropriately formatted to be parsed by the Date object.
If a.date and b.date are numbers
In the scenario where a.date and b.date are already represented as milliseconds-since-The-Epoch values, you can simply use them directly:
return b.date - a.date;
If a.date and b.date are instances of Date
If a.date and b.date are instances of Date, you can utilize the getTime method to retrieve their underlying milliseconds-since-The-Epoch values:
return b.date.getTime() - a.date.getTime();