Exploring Flow's 'missing indexer property' on class computed properties

flowtype-missing-indexer-property.js

/* @flow */

type KeyType = string
type ValueType = any

type MyMap = {
  [KeyType]: ValueType
}

const object: MyMap = {
  'foo': 'bar',
}
object['foo'] = 'baz'
object.foo = 'bork'

declare class A {
  [KeyType]: ValueType
}

const a: A = new A
a['foo'] = 'bar'

declare interface IMap {
  [KeyType]: ValueType
}

// eslint: Parsing error: Unexpected token, expected "{" (Fatal) (@ 'implements')
declare class B /*:: implements IMap */ {
  [KeyType]: ValueType
}

// fix eslint
declare class B /*:: implements IMap*/ {
  [KeyType]: ValueType
}

const b: B = new B
b['foo'] = 'bar'
b.foo = 'bork'	// flow: Cannot assign `'bork'` to `b.foo` because property `foo` is missing in `B` [1].

declare class C /*:: implements IMap*/ {
  [KeyType]: ValueType,
  foo: string,
  hello: () => void,
}

const c: C = new C
c['foo'] = 'bar'
c.foo = 'bork'
c.hello()