HTML Code
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>IndexedDB update if exists</title>
  <style>
    body { font-family: system-ui, sans-serif; margin: 1.5rem; line-height: 1.5; }
    button { margin-right: 0.5rem; margin-bottom: 0.5rem; padding: 0.5rem 0.9rem; cursor: pointer; }
    #status { margin-top: 1rem; }
  </style>
</head>
<body>
  <h1>Update only if exists</h1>
  <p>Try updating id 1 (exists after seed) and id 99 (missing).</p>

  <button type="button" id="seedBtn" disabled>Seed id 1</button>
  <button type="button" id="upd1Btn" disabled>Update id 1</button>
  <button type="button" id="upd99Btn" disabled>Update id 99</button>

  <p id="status">Opening database&hellip;</p>

  <script>
    var statusEl = document.getElementById('status');
    var db;

    function updateContact(id, changes) {
      var tx = db.transaction(['contacts'], 'readwrite');
      var store = tx.objectStore('contacts');
      var updated = false;

      store.get(id).onsuccess = function (event) {
        var row = event.target.result;
        if (!row) {
          statusEl.textContent = 'Skip: id ' + id + ' missing';
          return;
        }
        Object.assign(row, changes);
        store.put(row);
        updated = true;
      };

      tx.oncomplete = function () {
        if (updated) {
          statusEl.textContent = 'Updated id ' + id;
        }
      };
    }

    var openReq = indexedDB.open('tryitUpdateExists', 1);

    openReq.onupgradeneeded = function (event) {
      event.target.result.createObjectStore('contacts', { keyPath: 'id' });
    };

    openReq.onsuccess = function (event) {
      db = event.target.result;
      statusEl.textContent = 'Ready.';
      ['seedBtn', 'upd1Btn', 'upd99Btn'].forEach(function (id) {
        document.getElementById(id).disabled = false;
      });
    };

    document.getElementById('seedBtn').addEventListener('click', function () {
      var tx = db.transaction(['contacts'], 'readwrite');
      tx.objectStore('contacts').put({
        id: 1,
        name: 'Alex Rivera',
        email: 'alex@example.com'
      });
      tx.oncomplete = function () {
        statusEl.textContent = 'Seeded id 1.';
      };
    });

    document.getElementById('upd1Btn').addEventListener('click', function () {
      updateContact(1, { email: 'team@example.com' });
    });

    document.getElementById('upd99Btn').addEventListener('click', function () {
      updateContact(99, { email: 'nope@example.com' });
    });
  </script>
</body>
</html>
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IndexedDB Update — Only If Exists
