Make flow assume that a particular key does exist in a Map - flowtype

Question

Due to the way you're iterating over something, you may know that a key always exists in a Map but flow may not know.
Currently, how I'm getting rid of it is to check if the result returned from get() is not undefined.
I don't want to add those spurious checks though, and instead I want to tell the typesystem that it can assume that that key exists. Telling this to the typesystem also conveys more information (this key definitely exists) about the program to a future reader than a spurious check (this key may or may not exist, and if doesn't exist then we're ignoring that case)
So I want to be able to do the following by preferably adding something on or before line 3 instead of an if check.
1. let score:Map<string, number> = new Map()
2. initialize(score); // some magic initilization of the map that flow doesn't understand
3. score.get("hello");

Answer 1

I wouldn't recommend doing this but you could extend the Map class to define your own class that defines get,
declare class MyMap<K, V> extends Map<K,V> {
get(key: 'hello'): V;
get(key: K): V | void;
}
so that (new myMap()).get('hello') has type V instead of V | void. For example,
const myMap: MyMap<string, number> = new MyMap();
const n: number = myMap.get('hello');
const o: number = myMap.get('not hello'); // error
const p: number | void = myMap.get('not hello');
Try Flow
You'll need to change your code so that initialize returns a MyMap instead of modifying the passed in Map. Note that although this will pass Flow, it may still break during running. As you can see in the example above, even though I have created a new MyMap(), I have not added the key 'hello' and so n isn't really a number.