const request = indexedDB.open("library", 3); // Request version 3.
let db;

request.onupgradeneeded = function(event) {
  const db = request.result;
  if (event.oldVersion < 1) {
    // Version 1 is the first version of the database.
    const store = db.createObjectStore("books", {keyPath: "isbn"});
    const titleIndex = store.createIndex("by_title", "title", {unique: true});
    const authorIndex = store.createIndex("by_author", "author");
  }
  if (event.oldVersion < 2) {
    // Version 2 introduces a new index of books by year.
    const bookStore = request.transaction.objectStore("books");
    const yearIndex = bookStore.createIndex("by_year", "year");
  }
  if (event.oldVersion < 3) {
    // Version 3 introduces a new object store for magazines with two indexes.
    const magazines = db.createObjectStore("magazines");
    const publisherIndex = magazines.createIndex("by_publisher", "publisher");
    const frequencyIndex = magazines.createIndex("by_frequency", "frequency");
  }
};

request.onsuccess = function() {
  db = request.result; // db.version will be 3.
};