let book = {
  name: "Harry Potter 1",
  price: {
    value: 50,
    currency: "EUR"
  },
  ISBN: "978-7-7058-9615-2",
  weight: {
    version1: {
      value: 550,
      unit: 'g'
    },
    version2: {
      value: 690,
      unit: 'g'
    }
  }
}

if (book && book.weight && book.weight.version2 && book.weight.version2.value) {
  //Do something with book.weight.version2.value
  console.log(book.weight.version2.value);
}

if (book && book.weight && book.weight.version3 && book.weight.version3.value) {
  //Since there is no version 3, this block would not run and throw any errors
  console.log(book.weight.version3.value);
}

//Accessing without checking
console.log(book.weight.version3.value);
//error