type-guards.ts

export function isObj(arg: unknown): arg is Record<string, unknown> {
  return Object.prototype.toString.call(arg) === '[object Object]';
}

export function hasOwn<T = unknown>(arg: Record<string, unknown>, key: unknown): arg is Record<string, T> {
  return (
    isObj(arg) &&
    Boolean(key) &&
    typeof key === 'string' &&
    key.length > 0 &&
    Object.prototype.hasOwnProperty.call(arg, key)
  );
}

const a = { a: 1 };
if (hasOwn<number>(a, 'a')) {
  console.log(a.a); // a.a: number
}