What type do you use for an object that may contain anything, for example a JSON response from an endpoint?

The problem with any is that it allows unsafe access:

var a: any;
a.foo.bar; // What if .foo is undefined?

The problem with unknown and object is they don't allow any access

Enter UnknownObject. It allows getting arbitrary properties, but only safely:

var u: UnknownObject;
u.foo?.bar; // This is fine
u.foo.bar; // However, .foo might not be there, so this is a type error!