Get compat warnings in Vue 3 mode?

[thedamon]

Using @vue/compat it seems that when I turn on Mode: 3 I don't get behavior change warnings anymore. Is there any way to still see them? This would be far more useful to me as my components run, but I'm worried about small differences in the classes that are passed etc.

[LinusBorg]

If you run in mode: 3 you likely have already changed the behaviour to the ones that are supported in Vue 3, so what should it be warning about? Can you give a specific example of a warning you would like to get in mode: 3?

[Masa-Shin]

The migration guide says "If all compiler warnings are gone, you can also set the compiler to Vue 3 mode" so I thought it was okay to switch to mode 3 even though Vue Warnings remain in the console. Since the guide doesn't provide clear differences between mode 2 and mode 3, I'm not really sure when to use mode 3.

For example, I don't quite understand why the new COMPONENT_V_MODEL behavior isn't automatically opted in when the mode is set to 3. We need to explicitly set COMPONENT_V_MODEL: false to opt into the modelValue features, even in mode 3. This makes me question the real significance of 'mode 3'.

[LinusBorg]

The important part in your quote is "compiler warnings". The migration build has two "levels" of compat functionality:

1. compiler-based compat
2. runtime-based compat.

In the docs, we guide you to first fix all the compiler-related warnings and then switch the compiler-mode to 3 in the build-tool config.

Then the app uses the Vue 3 compiler for templates, but still provides runtime-compat for Vue 2 runtime APIs, for which you will continue to get warnings in the browser console.

These runtime compat options can be opted out per component or globally, per feature or as a whole, as documented a bit further down here:

https://v3-migration.vuejs.org/migration-build.html#global-config

COMPONENT_V_MODEL is a runtime compat setting, so switching the compiler to mode: 3 will not change this flag's behaviour, only a setting in the runtime config will.

[Masa-Shin]

Thanks! The facts there are two comapts and MODE doesn't impact run-time one made it clear for me.