`isReadonly` returns `false` for computed value (without a set function) nested in a reactive object

[cloydlau]

Vue version

3.5.11

Link to minimal reproduction

https://play.vuejs.org/#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

Steps to reproduce

1. Open the reproduction link
2. Check the Console

What is expected?

1. Computed value (without a set function) is readonly, so maybe those nested in a reactive object should also be readonly, right?
2. The PR below shows that mutating a readonly ref nested in a reactive object is not allowed, but it's not clear for this case
   fix(reactivity): mutating a readonly ref nested in a reactive object should fail. (close #5042) #5048

What is actually happening?

1. isReadonly(ro.c) returns false
2. ro.c = undefined throws TypeError: 'set' on proxy: trap returned falsish for property 'c'

System Info

No response

Any additional comments?

No response

[jh-leong]

This behavior is expected, see https://vuejs.org/api/reactivity-core.html#reactive:

A reactive object also deeply unwraps any properties that are refs while maintaining reactivity.

In your case, ro.c is a number not a computed, so isReadonly(ro.c) returns false

[cloydlau]

OK with the first point.

ro.c is a number not a computed

Then why ro.c = undefined throws TypeError: 'set' on proxy: trap returned falsish for property 'c'?

[jh-leong]

This is also related to Ref unwrapping.

When you try to update ro.c, you're effectively trying to update the computed value, which is not allowed, leading to the error.

[cloydlau]

I think it's kind of contradictory and confusing to have a variable that isReadonly returns false but doesn't allow it to be mutated. If you think this is in line with expectations, I'd personally recommend at least stating it in the documentation.

[jh-leong]

You're right, the current error message doesn't make it clear that this behavior is due to Ref unwrapping. I've submitted a PR try to improve the warning in #12141:

Before:

After: