Wannabe defaultdict for typescript. Not really tested.

defaultdict.ts

class DefaultDict<T, Q> extends Map<T, Q> {
  defaultFactory: () => Q
  constructor(defaultFactory: () => Q) {
    super()
    this.defaultFactory = defaultFactory
  }
  get(name: T): Q {
    if (this.has(name)) {
      return super.get(name)!
    } else {
      const value = this.defaultFactory()
      this.set(name, value)
      return value
    }
  }
}

How should this be used?

Haven't used this in ages. IIRC the same way as Map but it will insert and return a new entry when calling get with a key that isn't in the map.