Flow: Is it possible to add properties to Arrays?

[maranomynet]

I'm trying to add type annotations to an existing code and I have an Array instance that doubles as an object with custom properties set on it.

Like So:

const foo = [];
foo[0] = 13;
foo.push(42);

foo.superLevel = 'extreme'; // Flow complains about this

I thought I might declare a new type called SuperArray, something along the lines of this:

type SuperArray = Array<number> & {
  superLevel: string,
}
const foo: SuperArray = [];
// ...

(Example on Flowtype.org/try)

But that doesn't work either. I've Goggled and browsed through the docs, and failed to find an answer to:

A) Is this at all supported? and
B) if so, how do I annotate it?

[CodyReichert]

Also see: https://github.com/facebook/flow/issues/631

Not sure if there's any more recent info on that.

[maranomynet]

Yay! using $Supertype<> seems to work now:

https://flow.org/try/#0MYGwhgzhAED6DKBXADgUwE4EF3rAT2lQA8AXVAOwBMZtc8AecxAWwCMMA+aAbwChpoEFBgAyqAG6oQALkEl0AS3IBzANy8AvrxJ400JGiw580ALzQAJAYw609BMKN0OvXsAD25CCWgAzd+6y1k4m5gDaALrq-u5hAAwRZtAAjADM0QEAdMiIEAAWABQALABMAJTqvDGZQoZikiBJAOQxrGDoTeoA9F1+WbWiElJJyarQPdAAojju6ACEvEA

[mrkev]

For the record, this works:

type SuperArray = (Array<number> & {
  superLevel: string,
})
declare var foo: SuperArray

but this doesn't

type SuperArray = (Array<number> & {
  superLevel: string,
})
var foo: SuperArray = []

The bug here is that [] should be assignable to SuperArray since it's a super type of Array.