Can someone explain me how does the operation inside reduce method affect the object in outer scope?

I am looking into a code which is to strip some prefix url from object properties.

const strip2 = (object, path) => {
  const keys = [path];
  const pathArray = keys[0].split('.');

  pathArray.reduce((prevObj, key, index) => {
    if (!prevObj[key]) {
      return null;
    }
    if (Array.isArray(prevObj[key])) {
      return prevObj[key].map(item => strip2(item, pathArray[index + 1]));
    }
    if (index === pathArray.length - 1) {
      prevObj[key] = prevObj[key].replace('/hello', '');
    }
  }, object);

  return object;
};

strip2(
  {
    title: 'this is the title',
    menu: [
      {
        link: '/hello/contact',
        title: 'Contact Us',
      },
      {
        link: '/hello/faq',
        title: 'Faq',
      },
    ],
  },
  'menu.link'
);

\ it will return
{
  title: 'this is the title',
  menu: [
    { link: '/contact', title: 'Contact Us' },
    { link: '/faq', title: 'Faq' }
  ]
}

I tried to trace it and I can’t understand how does the replace method inside reduce affects the object in outer scope.