StorageManager.persisted()

The persisted() method of the StorageManager interface returns a Promise that resolves to true if box mode is persistent for your site's storage.

navigator.storage.persisted().then(function(persistent) { ... })

None.

A Promise that resolves to a Boolean.

if (navigator.storage && navigator.storage.persist)
  navigator.storage.persisted().then(function(persistent) {
    if (persistent)
      console.log("Storage will not be cleared except by explicit user action");
    else
      console.log("Storage may be cleared by the UA under storage pressure.");
  });