Vue JS: Computed value vs. watcher

Question

I have 3 Booleans at the start, and if one of these 3 values changes by the user (e.g. HTML checkboxes), I want to set changed = true.

data()
  return {
    boolean1:false, //may initally be true/false
    boolean2:true,  //may initally be true/false
    boolean3:false, //may initally be true/false
    changed: false  //no changes to start with
  };
},

How do I track these 3 values properly in Vue? My impulse would be to use a watcher, but I now read several times that a computed property is the better choice for such tasks. Unfortunately, they didn't provide examples for such a simple tracking task, so how would a computed property look like? So far I have this:

computed: {
  comp_settings_change: function(){
    // if booleans change, set to true, else stay at false.
    return true
  }
},

Your impulse is right, a watch is better because you only want to set a flag when an event fires — Dan
– Dan, Commented Dec 8, 2020 at 23:56
really depends on what you are trying to do, if you are just trying to detect if the booleans have changed from the initial state, then a computed would be simple, but you need to capture the initial state, that way if a boolean changes from false to true, then changed would be true, then if that boolean changes back to false, then changed would go back to false. — Keith Nicholas
– Keith Nicholas, Commented Dec 9, 2020 at 0:09

tony19 · Accepted Answer · 2022-07-15 23:05:47Z

2

A watcher is more appropriate than a computed prop in this case because the flag only needs to be set once.

You could use vm.$watch() on the Booleans, which returns a function to stop watching after the initial change, allowing you to create a one-shot callback:

export default {
  mounted() {
    const unwatch = this.$watch(
      () => [this.boolean1, this.boolean2, this.boolean3],
      value => {
        unwatch()
        this.changed = true
      }
    )
  },
}

demo

edited Jul 15, 2022 at 23:05

answered Dec 9, 2020 at 0:38

tony19

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3 Comments

Dominic Over a year ago

Great answer, thanks. So if I would wanted to watch the booleans constantly (not just set the flag once but on every change) I would use a computed value with this $watch function?

Dominic Over a year ago

Come to think of it, a computed property with $watch() would probably just be a watcher :)

tony19 Over a year ago

If you wanted to set the flag with every change to the Booleans, you could use the same solution but without unwatch().

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