Turn Map.prototype.has into a type guard

lib.es2015.collection.d.ts

const inventory = new Map([
  ['🍊', 3],
  ['🍌', 1],
  ['🍏', 15],
] as const);

if (inventory.has('🍊')) {
  const oranges: number = inventory.get('🍊');
}

declare global {
  interface Map<K, V> {
    has<T extends K>(key: T): this is { get(key: T): V } & Map<K, V>;
  }
}