ReactJS State and Local Variable

You could have it be a state variable, but that wouldn't make much sense since the state is meant to hold variables that relate to the current state of the UI. Also, state variables are meant to trigger a re-render if they are updated (with setState()).

Neither of these would make much sense for a timer ID to be stored in the state. In other words, it's not a state variable because:

1. The timerID doesn't express the representation of the UI in any way.
2. Updating the timerID shouldn't trigger a re-render.

The official docs actually mention this:

The state contains data specific to this component that may change over time. [...] If you don't use it in render(), it shouldn't be in the state. For example, you can put timer IDs directly on the instance.

Well, you can store timerID in state, but this is not a good practice, as you can read in the same documentation page:

If you don't use something in render(), it shouldn't be in the state.

Every time that you update your state via setState, render method is invoking. So if this data doesn't change UI, then calling render method doesn't change anything and it will be a waste.

That is why all the 'helper' data, such as API requests, e.t.c is better to store as component-object keys.

Is timerID in the assignment this.timerID = setInterval just a variable

Yes, timerID is a variable returned by the setInterval function. It also is a normal property of class Clock ....

Why is this not a state?

Use timerID in state brings no benefits. It is only another place from state to store a shared variable within your class. Storing it in state only makes your state bigger and more complex (which is harder to manage) since timerID does not take place for the final view. Or more simpler, timerID is not used within render, so we don't need to put it in the state.