Type Casting in TypeScript

printAddress.test.ts

import {Person, printAddress, MissingAddressError} from './printAddress';

describe('printAddress', () => {
  it('throws an error when person does not have an address', () => {
    const benny = {
      age: 34,
    } as Person;
    expect(() => {
      printAddress(benny);
    }).toThrowError(MissingAddressError);
  });
});

printAddress.ts

export type Person = {
  age: number;
  address: {
    city: string;
  }
}

export function printAddress(person: Person): void {
  if (!person.address) {
    throw new MissingAddressError();
  }

  console.log(person.address.city);
}

export class MissingAddressError extends Error {
  constructor() {
    super('Please enter an address.');
  }
}