In Vue 3, there are two main ways to observe changes to reactive data: watch and watchEffect. While both accomplish the same goal of re-running code in response to changes in reactive data, they differ in important ways.

In short, watch is a function that allows you to observe changes to specific reactive data and perform actions in response to those changes. On the other hand watchEffect is a reactive function that automatically tracks any reactive dependencies used inside it.

Vue’s watch
When using watch, you provide two arguments. A reactive dependency which changes we want to track and a callback function that will be executed whenever the dependency change. This callback function is passed two arguments: the new value of the data and the old value of the data.

The deep & immediate Options

The watch provides two optional options that you can use to customize its behavior: deep and immediate.

The deep option allows you to recursively watch all nested properties of an object. By default, watch only watches the reference of an object, not its properties. This means that if you change a property of an object, the reference to the object itself will not change, and therefore the callback function for watch will not be executed.

The immediate option allows you to execute the callback function immediately after the watcher is created. By default, watch does not execute the callback function when it's first created.

Consider the following example:

<template>
  <main>
    {{ product.price }} = {{ product.quantity }} * 10
    <input v-model="product.quantity" type="number" />
  </main>
</template>

<script lang="ts">
export default {
  name: "ProductComponent",
  data() {
    return {
      product: {
        price: 10,
        quantity: 1
      }
    };
  },
  watch: {
    product: {
      handler: function (newVal, oldVal) {
        this.product.price = newVal.quantity * 10;
      },
      deep: false,
      immediate: true
    }
  }
};
</script>

We want to compote the product.price value based on the product.quantity chosen by the user. The watch function will run immediately and the product.price will be 10. However the watch function won’t run ever again, because it is dependent on the whole product object and we don’t change its reference.

To solve the above problem, use deep: true, or you can refactor the watcher to be dependent on the quantity field, such as:

watch: {
    "product.quantity": {
      handler: function (newVal, oldVal) {
        this.product.price = newVal * 10;
      }
    }
  }

Using Composition API, we no longer need to use the deep option, since we can make the watcher observing the refs value-change, such as:

<template>
  <main>
    {{ product.price }} = {{ product.quantity }} * 10
    <input v-model="product.quantity" type="number" />
  </main>
</template>

<script setup lang="ts">
const product = ref({
  price: 10,
  quantity: 1
});

watch(product.value,
  (newVal, oldVal) => {
        product.value.price = newVal.quantity * 10;
  },{ immediate: true });
</script>

Vue’s watchEffect

Unlike watch, watchEffect does not require you to specify the data that you're watching. Instead, it automatically tracks any reactive dependencies used inside it and re-runs the code whenever any of those dependencies change. We can rewrite the above example with the following syntax:

<template>
  <main>
    {{ product.price }} = {{ product.quantity }} * 10
    <input v-model="product.quantity" type="number" />
  </main>
</template>

<script setup lang="ts">
const product = ref({
  price: 10,
  quantity: 1
});

// runs every time its dependencies (quantity) change
watchEffect(() => {
  product.value.price = product.value.quantity * 10;
});
</script>

Usage

In the above example, we used watch or watchEffect to calculate the price value based on the quantity change. For examples like this, it is preferable to use computed property, instead of watchers. Watchers are used mainly for side effects and common use cases may be:

• Making an API call
• Saving the changed state of the dependency into some local storage
• Redrawing a chart based on changed dependencies
• Tracking user behavior

Summary

This blog post explores the differences between Vue's watch and watchEffect functions, which both allow you to observe changes to reactive data. watch is used to observe specific reactive data and perform actions in response to those changes, while watchEffect automatically tracks any reactive dependencies used inside it.

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