That would work at "compile" time, but how do we know whether or not a property is missing at runtime without accessing it? Or resorting to an expensive and min

That would work at "compile" time, but how do we know whether or not a property is missing at runtime without accessing it? Or resorting to an expensive and minifier-hostile call such as hasOwnProperty? The TypeScript approach won't work here. Guaranteeing that optional properties have a default value would be one solution, another would be to use ES6 default parameters instead of options objects. But there has to be some sort of value to use at runtime if missing property access always throws.