TypeScript reporting Object is possibly 'undefined' when it shouldn't

Question

I\'m trying to understand why TypeScript is giving me the following error: Object is possibly \'undefined\'

Here\'s the code snippet:

Answer 1

If you query for a key not in the map, you will get undefined. Not because the map holds an undefined value, but because it does not hold a value at all.

Try

results.get(key)!

Or

results.get(key) as number

Answer 2

Becouse the compiler is not so smart to take into account behavior of Map.

Answer 3

There is no type guarding for Map.has() See this issue or this issue.

You can fix the issue by having a fallback of 0, -1 or whatever you think would be best for your code.

const mergedDepsObj: { [key: string]: string } = {}
const results: Map<string, number> = new Map();
Object.keys(mergedDepsObj).forEach((key: string) => {
  results.has(key) ? results.set(key, (results.get(key) || 0) + 1) : results.set(key, 0);
})

Custom Type Guard

However, above being said, we make our own type guard with help from this comment.

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

    has<CheckedString extends K>(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;
}

const mergedDepsObj: { [key: string]: string } = {}
const results: Map<string, number> = new Map();

Object.keys(mergedDepsObj).forEach((key: string) => {
    results.has(key) ? results.set(key, results.get(key) + 1) : results.set(key, 0);
})

We can see it in action here