ES6 adds default and rest parameters to your functions. Default parameters can replace the old method of having to check whether arguments were passed in, then assigning them if they're missing:
var production = true;
// By default, use log. Before we'd have to check argument.length
// or whether type was undefined to get the same effect:
function DEV_LOG(string, type = "log")
if (!production || type === "error")
// Won't print since in production.
// Will print since its an error.
DEV_LOG("Error Log", "error");
With rest parameters, you can easily treat multiple parameters as if they were part of a list: