[Haegin]

> avoids changing-state and mutable data.

This doesn't mean that functional programming eliminates state. Avoiding changing-state and mutable data is different and the Wikipedia article is referring to how functional programming doesn't mutate existing data, so you avoid the stale reference problems that can occur in OO languages.

Instead, the state is the current execution state of the program. Function calls are side affect free (except when interacting with the external world, which is a special case I'm not covering here). Because of this, the only way data can be communicated from one function to another is by passing it in when calling the function, or by returning it. This means the state is just the data local to the currently executing function, and any calling functions (though the data in that part of the state isn't available to the current function it's still in memory until the calling function returns).

Contrast this with procedural programming languages, where state can also be maintained in global variables, or object oriented languages, where objects maintain their own state with the system state being spread around the whole system.