Persistent Storage

I use Inexed-DB in ionic3 application in some cases, some IDB nodes are deleted by the userAgent. in MDN, i found a method , that persists the storage and prevents the userAgent from deleting in case of storage pressure

I try to run this in my ionic 3 code like:

declare var navigator; {
      if (persistent)

but i got always


Is ther any way to persist it (such as permissions or any thing else)

these links may help:
Chrome Persistnet storage

StorageManager.persist() Reference