[HTML 5] Back Forward Cache

Blog: https://web.dev/bfcache/

bfcache is an in-memory cache that stores a complete snapshot of a page (including the JavaScript heap) as the user is navigating away. With the entire page in memory, the browser can quickly and easily restore it if the user decides to return.

How many times have you visited a website and clicked a link to go to another page, only to realize it's not what you wanted and click the back button? In that moment, bfcache can make a big difference in how fast the previous page loads:

Observe when a page is restored from bfcache

window.addEventListener('pageshow', (event) => {
  if (event.persisted) {
    console.log('This page was restored from the bfcache.');
  } else {
    console.log('This page was loaded normally.');
  }
});

window.addEventListener('pagehide', (event) => {
  if (event.persisted) {
    console.log('This page *might* be entering the bfcache.');
  } else {
    console.log('This page will unload normally and be discarded.');
  }
});

No "unload"

Instead of using the unload event, use the pagehide event. The pagehide event fires in all cases where the unload event currently fires, and it also fires when a page is put in the bfcache.

"beforeunload"

Don't

window.addEventListener('beforeunload', (event) => {
  if (pageHasUnsavedChanges()) {
    event.preventDefault();
    return event.returnValue = 'Are you sure you want to exit?';
  }
});

Do:

function beforeUnloadListener(event) {
  event.preventDefault();
  return event.returnValue = 'Are you sure you want to exit?';
};

// A function that invokes a callback when the page has unsaved changes.
onPageHasUnsavedChanges(() => {
  window.addEventListener('beforeunload', beforeUnloadListener);
});

// A function that invokes a callback when the page's unsaved changes are resolved.
onAllChangesSaved(() => {
  window.removeEventListener('beforeunload', beforeUnloadListener);
});

Handle IndexDB

let dbPromise;
function openDB() {
  if (!dbPromise) {
    dbPromise = new Promise((resolve, reject) => {
      const req = indexedDB.open('my-db', 1);
      req.onupgradeneeded = () => req.result.createObjectStore('keyval');
      req.onerror = () => reject(req.error);
      req.onsuccess = () => resolve(req.result);
    });
  }
  return dbPromise;
}

// Close the connection to the database when the user is leaving.
window.addEventListener('pagehide', () => {
  if (dbPromise) {
    dbPromise.then(db => db.close());
    dbPromise = null;
  }
});

// Open the connection when the page is loaded or restored from bfcache.
window.addEventListener('pageshow', () => openDB());