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

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 = 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 {
    // Works if there are other known strings.
    has(this: MapWith, key: CheckedString): this is MapWith

    has(this: Map, key: CheckedString): this is MapWith
}

interface MapWith extends Map {
    get(k: DefiniteKey): V;
}

const mergedDepsObj: { [key: string]: string } = {}
const results: Map = 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