Test if we can modify IndexedDB records (not upgrade the DB version) in one connection while another connection is open. Spoiler, we can!

README.md

To run this test, copy and paste the script into the JS console of your browser (or use Snippets in Chrome devtools).

The output is:

starting
upgrading
opened1
req1 success
req2 success
req3 success
req4 success
opened2
cursor continue
cursor continue
delete success for 2
cursor continue
cursor continue
delete success for 4
closing2
closing1
done

doit.js

runTest(
  'test' + Date.now(),
  1,
  'requests',
).then(console.log)

function runTest(dbName, dbVersion, objectStoreName) {
  return new Promise((resolve, reject) => {
    console.log('starting')
    const connection = indexedDB.open(dbName, dbVersion)
    connection.onupgradeneeded = function(event) {
      console.log('upgrading')
      const db = event.target.result
      db.onerror = console.error
      const objectStore = db.createObjectStore(objectStoreName)
    }

    connection.onsuccess = e => {
      console.log('opened1')
      const db = e.target.result
      const transaction = db.transaction([objectStoreName], 'readwrite')
      const objectStore = transaction.objectStore([objectStoreName])
      objectStore.add({ blah: 1 }, 1).onsuccess = function() {
        console.log('req1 success')
      }
      objectStore.add({ blah: 2 }, 2).onsuccess = function() {
        console.log('req2 success')
      }
      objectStore.add({ blah: 3 }, 3).onsuccess = function() {
        console.log('req3 success')
      }
      objectStore.add({ blah: 4 }, 4).onsuccess = function() {
        console.log('req4 success')
        modifyWhileOpen(dbName, dbVersion, objectStoreName).then(() => {
          console.log('closing1')
          db.close()
          resolve('done')
        })
      }
    }
  })
}

function modifyWhileOpen(dbName, dbVersion, objectStoreName) {
  return new Promise((resolve, reject) => {
    const conn = indexedDB.open(dbName, dbVersion)
    conn.onsuccess = e => {
      console.log('opened2')
      const db = e.target.result
      const transaction = db.transaction([objectStoreName], 'readwrite')
      const objectStore = transaction.objectStore([objectStoreName])
      const request = objectStore.openCursor()
      request.addEventListener('success', e => {
        const cursor = e.target.result
        if (cursor) {
          const val = cursor.key
          if (val % 2 === 0) {
            const delReq = cursor.delete()
            delReq.onsuccess = () => {
              console.log('delete success for ' + val)
            }
          }
          console.log('cursor continue')
          cursor.continue()
          return
        }
        console.log('closing2')
        db.close()
        return resolve()
      })
    }
  })
}