Flow analysis doesn't work with es6 collections 'has' method

TypeScript Version: 2.1.1

Code

const x = new Map<string, string>();
x.set("key", "value");
if (x.has("key")) {
  const y : string = x.get("key");  // error: y is string | undefined, not string
}

Expected behavior: y is narrowed down to string Actual behavior: y is still string | undefined even after checking if the map has that key

[ivstas]

@DanielRosenwasser I think you’re a bit overcomplicating the stuff.

const map = new Map<string, string>();
const value = <string>(map.has('key') ? map.get('key') : 'default value');

Anyway, these all are workarounds. What’s need to be fixed if flow analysis.

[DanielRosenwasser]

Actually, you can get this to work better with another overload.

const x = new Map<string, string>();

interface Map<K, V> {
    // Works if there are other known strings.
    has<KnownKeys extends string, CheckedString extends string>(this: MapWith<string, V, KnownKeys>, key: CheckedString): this is MapWith<K, V, CheckedString | KnownKeys>

    has<CheckedString extends string>(this: Map<string, V>, key: CheckedString): this is MapWith<K, V, CheckedString>
}

interface MapWith<K, V, DefiniteKey extends K> extends Map<K, V> {
    get(k: DefiniteKey): V;
    get(k: K): V | undefined;
}

x.set("key", "value");
if (x.has("key") && x.has("otherKey")) {
  const a: string = x.get("key"); // works!
  const b: string = x.get("otherKey"); // also works!
}