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not sure the distinction between null and undefined is doing anything for me here
Depends on the programming language. In JavaScript, it literally means that like the key or variable does not actually exist. Whereas like in C/C++, writing random bytes to random memory addresses would result in "undefined behaviour" which means basically anything could happen.
In Javascript you can do
let a = undefined, defining the varialeaasundefined.A significant difference to defining it as
nullis thattypeof null == "object", whiletypeof undefined == "undefined".
Basically JavaScript uses undefined to mean keys that don't exist. You know how sometimes when you're wondering about the semantics of "present but null" and "absent"? It's basically that. Undefined means it isn't there but things are only null if they've been set to null.
There are probably more nuances but that is the gist.