newGreeter.ts

function checkLanguage(greeter, closing){
  if (closing) {
    const newGreeter = clone(greeter);
    newGreeter.greeting = 'Goodbye';
    return newGreeter;
  }
  return greeter;
}

const foo = { greeting: 'Hello' };
const newFoo = checkLanguage(foo, true);
console.log(newFoo.greeting + ' world!');