Vue 3 Composition API - watchEffect Vs watch

watchEffect seems to be a simplified watch and the main differences are

• Only accepts a function
  • watch can accept either a function or one or more reactive properties.
• Runs immediately when defined and when reactive dependencies change
  • watch only runs when reactive dependencies change

I would use:

• watchEffect when I want to watch multiple reactive properties and I don't care about old values
• watch when I want to watch one specific reactive properties and I may want old value

Note, above is what I would use them for, but may not be their only usage.

Also found in docs regarding the difference:

Compared to watchEffect, watch allows us to:Perform the side effect lazily;Be more specific about what state should trigger the watcher to re-run;Access both the previous and current value of the watched state.

Source: https://composition-api.vuejs.org/api.html#watch

watchEffect is something introduced in Vue3 with its composition api. The reason to have both watchEffect and watch, as I understand, is to keep the semantics of watch as close as possible to that of Vue2. The birth of watchEffect, if you are interested, can be traced back to here and here

As it stands today, watchEffect is an immediate/eager watch that uses a more concise/consistent syntax (consistent with computed):

1. watchEffect does not accept explicit watch sources, but instead automatically figures out all the dependencies by immediately executing the callback (or effect as how it is called in the source code), similar to how computed works. Therefore watchEffect must run the effect immediately. And because of this, there is a common trap (at least I have to keep reminding myself of it constantly) when setting up watchEffect: you need to make sure that during the first execution of your watchEffect, all of the dependencies are indeed accessed. How would some dependency escape from being accessed? Watch for conditional statements.
2. watchEffect will run its effect immediately as mentioned above.
3. watchEffect is a deep watch. This is something I am not sure whether it is intended or not. If you use a reactive object inside your effect, any change on that object will cause the effect to rerun, even if the changed property is not the one you accessed or is nested.

If Vue 3 is designed from scratch or there is no concern of maintaining backward compatibility, I would imagine there will only be watchEffect