Calling IDBObjectStore.put(object) resulting in a constraint violation, should leave the database as if the operation did not take place, but if the operation fails due to a violation of a unique index, the end result will be a deletion of the object.

Repro link: https://jsitor.com/0oBx-UC93

The repro has an object store "users" with primary key "id" and unique index on property "username".
First, it adds two objects in the object store:

{id: 1, username: "foo"}
{id: 2, username: "bar"}

Then, it does a put operation with {id: 1, username: "bar"}, which would violate the uniqueness of the username index because "bar" is already occupied by the second object (with id 2).

After that transaction has ended (auto-committed), the repro will list all users in the database and only find the second entry.