Add/map key dom wrapper maps 9

I have to say - moving to ES6 is going to confuse me for a while. I always expect JS objects .prop to behave without any sideeffects (like building a json object programmatically). While I know of https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineSetter__ I've always considered it automagical dangerousness. So this confused me - I thought 'why the hell is he checking that dot notation works?' I must say I'm not a huge fan of may or may not exist setters/getters at the moment. Does ES6 through some kind of exception if you try to set an undefined property of a class? Like key.tite = 'abc' ?