fallback to default version in migrations

[levsero]

Related issue/comment: #434 (comment)

What's changed?

Currently if you try and upgrade from v4 to v5 using https://github.com/rt2zz/redux-persist/tree/b85b56a889ebd4919c9ba11d239a4ae1861e1da6#experimental-v4-to-v5-state-migration and also add a migration, the migration will attempt to run on the v4 state which has no _persist version.

This pr would allow the migration to fall back on the default version in such a scenario.

Considerations

• I do not believe this would affect the code in any other situation as regularly this won't be executed without having the _persist.version.
• Is there a clearer or cleaner way to tie this code specifically to using the v4 migrate. eg. perhaps we can manually add _persist in the v4 getStoredState so the v4 migration will not affect any other parts of the code?

[rt2zz]

@levsero interesting, glad you are testing out the v4->v5 state migration! Is it working otherwise?

I think agree with defaulting state version here. Since the code relies on version existing, if it were to not exist for any reason (even though this should never happen) we want the migration to still make sense.

[rt2zz]

actually I just realized the this default to default version if state version === 0 which i incorrect. Will need to update

[rt2zz]

updated to include an undefined check 👍

[levsero]

Good catch on the === 0 👍
Haven't come across any other issues in the v4 -> v5 migration, seems to work as intended.
💯 on all the work you've done here.

[levsero]

@rt2zz wondering how soon do you plan on cutting another release? (Would like to make use of this asap in production and would prefer not having to fork anything.)

[rt2zz]

I cut a release under the next tag (5.3.0-rc) with this change as well as a change to support non-object substate. I would say the changes are fairly safe so if you are able to give it a go and confirm we are 👍 I will cut 5.3.0 proper.

[levsero]

@rt2zz can confirm that on 5.3.0 the migrations (both from v4-v5 as well as the state migration) all appear to be working correctly 🙇