I think this was intentional. null is a special value, and should not be replaced with default. If you want default, then send in undefined.

/cc @phanan, I recall you mentioning it.

Thanks @simplesmiler. I wondered if that might have been the case. Gotta love breaking changes :)

Not sure how I would send in undefined. Currently I send in either a json_encoded php array, or null. I can change null to something else but how would I send in undefined? If I send as a string it fails as the prop expects an array. Plus it would compile as a string anyway, even if it didn't fail. I could send an empty array but then it obviously stays as an empty array and doesnt fallback to the default prop value.

I feel that null would have made more sense when bridging from php to js...

Something like :prop="undefined", just like you did with :prop="null".

Ok thank you. Seems it has to be explicitly set as opposed to being set from a php callback. Its being sent as a string in that latter case which is causing Vue to squawk. In the former (like your example above) it works fine and is recognised as JS undefined.

FYI, the previous misbehavior (treating null as undefined) was fixed in this commit: 4d4059b

<myComponent :value="foo || undefined"/> :)