Should the parameter (currentValue) in the watch function be mutable?

[cangSDARM]

consider we have a simple sample:

const values = ref({});

const verify = (data) => {
   data['some-temp-metadata'] = 'xyz';

  // ...
}

watch(values, verify, { deep:true });
onMounted(() => {
   values.value = {...DEFFAULT_VALUES};
});

In this sample, the values will change twice, and the watch sideffect verify will trigger twice. The second trigger is because, inside the watcher, we changed data['some-temp-metadata'].

Is this correct behavior (i know we should avoid it, but it exists), or should it be banned by making currentValue immutable?

[xuyimingwork]

1. I think this is not a correct behavior.
2. I think making currentValue immutable will cause extra transform in runtime.

So, maybe a eslint rule can solve this?

[skirtles-code]

This appears to be working correctly to me.

Watchers are allowed to trigger recursive updates. This isn't an accident, there's specific code to ensure it works:

core/packages/runtime-core/src/apiWatch.ts

Lines 212 to 216 in 75220c7

	// important: mark the job as a watcher callback so that scheduler knows
	// it is allowed to self-trigger (#1727)
	if (cb) {
	job.flags! |= SchedulerJobFlags.ALLOW_RECURSE
	}

Recursively updating state like this is valid usage, and the callback is responsible for ensuring that the watched state eventually stabilises so it doesn't recurse indefinitely.

Updating the property via data is equivalent to updating it via values.value, they both refer to the same object. I think updating via values.value may be slightly clearer, but I don't see any reason to prevent updates via data.

I'm not sure what form immutability would take in this context, but I would also note that attempting to make it immutable by wrapping it in a proxy would break equality checks within the callback, so that isn't really a viable approach.

[cangSDARM]

However, this usually involves some unintended bugs. If this is a necessary design, I think there should be at least a warning provided for cases where triggers occur through direct modification of currentValue.