SafeMap.ts

// ----- Definition -----

class SafeMap<V> {
  private readonly o: { [key: string]: V } = {};

  public put(key: string, value: V): void {
    this.o[key] = value;
  }

  public has<K extends string>(key: K): this is SafeMapWithKey<K, V> {
    return key in this.o;
  }

  private get(key: string): V {
    return this.o[key];
  }
}

interface SafeMapWithKey<K extends string, V> {
  get(key: K): V;
}

// ----- Usage -----

const map = new SafeMap<number>();

// TypeScript error: Property 'get' is private.
console.log(map.get("hello"));

// Typechecks fine.
if (map.has("hello")) {
  console.log(map.get("hello"));
}