Alvin Crespo

Today I was trying to figure out why a passed in prop that I was setting on data for a component wasn’t being set.

Check this out:

// my-component.vue

export default {
  props: ["rating"],
  data() {
    return {
      rating: this.rating,
    }
  },
  computed: {
    normalizedRating: function() {
      return this.rating * 100
    },
  },
}

At first glance, this looks great right? Well, not necessarily. It depends on rating is being loaded.

If it’s an asynchronous call to your API - this code won’t work. When the component is mounted, this.rating will be undefined.

To fix this, we need to add a watcher:

// my-component.vue

export default {
  props: ["rating"],
  data() {
    return {
      rating: this.rating,
    }
  },
  watch: {
    rating: {
      deep: false,
      handler(val) {
        this.rating = val
      },
    },
  },
  computed: {
    normalizedRating: function() {
      return this.rating * 100
    },
  },
}

🎆 It now works! This is because when the parent component finally loads in the rating - our component will react to it.

So, whats a watcher anyways?

Watch an expression or a computed function on the Vue instance for changes.

vm.$watch API

TL;DR - the handler will get called when this value changes.

The guide states:

In this case, using the watch option allows us to perform an asynchronous operation (accessing an API), limit how often we perform that operation, and set intermediary states until we get a final answer.

Watchers Guide

Alright! Thank you for reading my learnings from today. Did you learn something new? Anything here that I got wrong or can improve on? Let me know at @alvincrespo.

Cheers!