TypeScript Is Amazing

sparse-map.ts

//
// WRONG
///

class InMemorySparseMap<K, V> implements SparseMap<K, V> {
  private latest: Map<K, Updatable<V>>;
  private factory: ((key: K) => Observable<V>) | undefined;  

  subscribe(key: K): Updatable<V> {
    let ret = this.latest.get(key);
    if (ret) return ret;

    if (this.factory) {
      // *** TypeScript reports this.factory can be null!! WTF!!! ***
      ret = new Updatable<V>(() => this.factory(key));
    } else {
      ret = new Updatable<V>();
    }

    this.latest.set(key, ret);
    return ret;
  }
}

// BUT
// The closure captures *this*, not *this.factory*.
// SO
// this.factory could be changed *later* to be undefined
  
//
// Right version!
//
  
    if (this.factory) {
      // Capture this.factory at the time of the call
      const fact = this.factory.bind(this);
      ret = new Updatable<V>(() => fact(key));
    } else {
      ret = new Updatable<V>();
    }