Difference between calling watcher with parentheses vs without

Question

In watch property, what is the difference between calling property using () and not using them?

export default {
  watch: {
    // syntax with ()
    test() {
      if (this.test) {
        //some work
      } else {
        //some work
      }
    },

    // syntax without ()
    labels: {
      immediate: true,
      deep: true,
      handler(option) {
        //some work
      }
    }
  }
}

Futhermore what is the aim of immediate and deep?

Answer 1

watch is lazy by default, meaning it would only run after an initial change to the source has occurred.

adding immediate: true essentially let's the callback of the watch to run when the component is initialized. In essence, like the do block of a do while loop in certain languages.

you can find more and the deep and eager watch at the docs https://vuejs.org/guide/essentials/watchers.html