tryflow | Try a static type checker for JavaScript | Runtime Evironment library

You need to generically type your baseID function so Flow knows what you expect as argument and return type. It seems like Flow doesn't use the type of IndentityFunction when trying to figure out what the baseId function is really doing.

(Try)

They are checked when you try and create a component, i.e:

Since Bar's on function is overriding Foos, the type of foo in the subclass's method needs to be a subtype of the corresponding type in the superclass's method. While 'bar' is a string, all string are not 'bar'.

For example, these will compile fine:

Of the three possible workouts you gave, I've figured out how to make it work using an single options object, however because you require at least one object to be set, you need to define each possibility.

Like this:

To access a member that is not common to all types in a union, you have to give flow a way to see which type it's dealing with. I understand that you're trying to use the truthiness of error to determine the specific type, but flow does not understand this.

The pattern flow uses for this situation is called "disjoint unions". It is described here: https://flow.org/en/docs/types/unions/

I'm not an expert on flow, but it seems one solution would be to modify your type definition for SuccessResponse as follows:

That would be unsafe. If something has type string at runtime, it doesn't mean that it has the same static type, e.g. it could be some enum: 'Foo' | 'Bar', so making it just string would allow unsafe mutations. On the other hand, it could be number | string, so in the future head could become a number or any type, really.

Instead you can do the following: