propCheck | Property based testing for kotlintest ported from quickcheck | Testing library

QuickCheck comes bundled with a NonEmptyList newtype, whose Arbitrary instance only generates non-empty lists. You can combine this with NonZero to get a non-empty list of non-zero integers:

You are NOT modifying the state before render, because you are using a setTimeout. Render does not wait for your setTimeout and is directly called after componentWillMount is called.

You have to set some default values in the redux reducer. You can just set in the reducer

This is completely normal. The attribute you see in the source is the original state of the element. The underlying property - which is what prop() gets/sets - can be different.

Here's an example:

Why not just use a simple function to check, and then use a different property name if the check returns true?

Because that would mean everyone needed to do these checks. And there needed to be a protocol on how to use a different property name.

Also, one needs to assume that existing code already uses all possible property names. If you want to introduce a new, standard method name to use in an iteration protocol, which one could you choose? There is no name that you can pick where it is assured that it doesn't collide with anything.

I'm guessing the "real" use of symbols is that of a tool to help those connected to the standards body implement new features without breaking preexisting code.

No, you don't have to be connected to the standards body to use symbols. Everyone can introduce his own property names that don't clash with other code. They're useful for all kinds of libraries that need to work with arbitrary objects and don't want to break encapsulation.