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 explain me what does the + symbol mean in front of the property in the code below:
export type User = {
+name: string,
+surname: string,
+personId: PourceId,
}
I could not find any reference in the documentation, any link is also welcome.
The + symbol in front of the property means the property is read-only
Reference: https://flow.org/en/docs/types/interfaces/#toc-interface-property-variance-read-only-and-write-only
The '+' symbol means the property is read only and
'-' means the property is write only and
If does not have any '+' or '-' symbol it means the property have both read/write access.
It can be used while defining interface property or type property.
The documentation can be found in below link:
https://flow.org/en/docs/types/interfaces/#toc-interface-property-variance-read-only-and-write-only
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