Boolean v-model doesn't default to undefined when not set

[CernyMatej]

Vue version

3.3.13

Link to minimal reproduction

https://play.vuejs.org/#eNp9UstugzAQ/BXLF1opj0NuEYnUVDm0Uh9qevSFwIaSGtuyF5oK8e9dm0BTNeLE7O4wO7PQ8DtjZnUFfMljl9rCIJOJyleCoxOcOcDKrIVKtXLI3JPOQLIVy+BQKAhV7NAWKl/fRB2Ibnv2/hp7r7WERBH9jDw/nneraREVCKWRCQJVjMVZUQdA0BvxgLFdWMXqaRlWKE3eAJesaRh+G9CH3mrbsimLqD3U0Vls7tX+CW86U2PK51iD8lD/UfbPzno8vwjEJ3RXOs+hyGdHpxWdvfFkwVNdmkKCfTFY0PkEp52doOCJlPrrMfTQVjDp++kHpJ9X+kd38j3BXy04sDUIPswwsTlgN97unuFEeBhS6EoSe2T4Bk7LynvsaJtKZWT7ghfcPpRGW6SP9O62JwTl+lDeqGe2gS84/Xv3I9F/7S5mi/CeUC1vfwBRQuR4

Steps to reproduce

1. Define a model with boolean type in a component
2. Do not specify the v-model when using the component in the template of a parent component

What is expected?

Since all other types have undefined value when they're not set, I would expect a boolean v-model to behave the same.
I assume the root cause of this problem is that normal boolean props are converted to false when not set.

I can see why this behavior might be desired for props in certain situations. However, it creates issues in detecting whether a v-model has been set. Therefore, I think it would be better to have consistency in prop behavior.

What is actually happening?

A v-model of boolean type has a value of false when not set, instead of undefined.

System Info

No response

Any additional comments?

No response

[LinusBorg]

This is documented behavior so 1) it's not a bug and 2) will not be changed in Vue 3.

The Boolean absent props will be cast to false. You can change this by setting a default for it — i.e.: default: undefined to behave as a non-Boolean prop.

[CernyMatej]

I know it's documented but I think the use-case for boolean props is different than boolean v-model. Wouldn't you at least consider adding the default value of undefined to the defineModel() macro so that it is consistent with other types of v-model?

[LinusBorg]

You can define a default in defineModel() just like you can for the normal Boolean prop. I don't think it's desirable to switch the behavior for defineModel<boolean>() only, that will only cause more confusion.

[matthew-dean]

@CernyMatej This has been a very-long-standing error in Vue, but it's probably too risky to correct it now.