Flow type, What does the `+` symbol mean in front a property?

Question

I came across the following code written in js FlowType (I am interested to know the value of + in the context of FlowType not in general JS).

Could you please expla

Answer 1

After some additional research I found out that +/- indicate the covariant or contravariant for a property.

interface MyInterface {
  +covariant: number;     // read-only
  -contravariant: number; // write-only
}

Interesting article on variance:

https://flow.org/en/docs/lang/variance/

https://flow.org/en/docs/types/interfaces/#toc-interface-property-variance-read-only-and-write-only