Flow: Feature request: read-only annotations/warnings

[eggyal]

It'd be great if Flow could warn if one tries to overwrite a variable that has been declared to be "read-only".

For example, suppose the ! character was introduced into type declarations to indicate "read-only" state; one could then do:

////////// Foo.js //////////
export default class Foo
{
    bar: !string;              // read-only property

    constructor(bar: !string)  // read-only parameter
    {
        this.bar = bar;        // read-only properties may be assigned in constructor
                               // (or perhaps from any class method)

        bar = 'quz';           // Flow violation
    }
}


////////// main.js //////////
import Foo from './Foo';
const foo = new Foo('qux');
foo.bar = 'quz';               // Flow violation

[avikchaudhuri]

Yeah, something like this is very much on the todo list. Will probably use const though.

[iamchenxin]

@avikchaudhuri Will this be landing on v0.37? Currently(0.36), seems const can not be used inside class .

[graue]

Flow 0.34 added covariant properties, which are _kind of_ like read-only properties. Not exactly the same. But it can help for some use cases. Check it out in the docs here: https://flowtype.org/docs/variance.html

[acertain]

Is it possible to use covariant properties and intersection types to get read-only? {x : +a} & {x: a} seems to do the right thing.

[calebmer]

You can use covariant properties now on classes 👍