Understanding watchers in Vue – LogRocket Weblog


Web Development

Vue is designed to be dynamically extensible, allowing you to immediately create reusable and maintainable parts, use client-aspect info binding, and offer a rich ecosystem of plugins to enhance its functionality.

A single of the numerous features readily available in Vue is the watcher purpose, which makes it possible for us to monitor an software state and set off actions primarily based on these variations. In this write-up, we’ll search at how watchers function in Vue, what they can do, and how to benefit from them to build effective apps.

What is a watcher?

A watcher in Vue is a particular element that allows us to notice some facts and execute certain steps when it modifications. It is a additional generic way to observe and react to info variations in the Vue instance.

With watchers, we are not just in a position to observe a home and carry out custom made steps we can also obtain the aged worth that this individual house is modifying from, as perfectly as the new benefit that it has adjusted to.

Making use of watchers

Applying the Options API, we may possibly employ the observe possibility to hear for improvements in a home benefit, as demonstrated beneath:

Also, we can also attain the same point with the Composition API‘s check out() purpose:

So that whenever the value of concept alterations, we are in a position to accessibility its new benefit or preceding worth and accomplish any most popular action.

In a subsequent chapter, we’ll appear at a lot more practical examples of utilizing watchers in serious-environment apps. Having said that, before we get there, let us go around all of the alternatives obtainable to a watcher.

Deep looking at with the deep solution

The default actions of watchers in Vue.js is shallow i.e., they only keep an eye on alterations to data at the leading stage and would not react to nested residence modifications.

For case in point, we have an array of objects:

…and set up a watcher for this data as we did above. Our watcher will not be brought on any time an object’s residence improvements, these types of as when we established someData[0].id to a random quantity, for case in point.

We can correct this by environment an optional deep property in our watcher to accurate. Nevertheless, this will adjust the syntax in which our watcher is designed a bit, as we’ll have to introduce a new handler purpose:

// . . .
    handler(newValue, oldValue) 
      // console.log(newValue, oldValue)
      console.log("someData transformed!")
    deep: real,

And with this new addition, our watcher will be triggered even when a nested property changes.

This instance can also be replicated with the watch() operate in the Vue 3 Composition API like down below:

The instant alternative

Watchers are not activated straight away unless the price you are observing has improved. But in some situations, we may perhaps want to complete certain actions with the initial price of the home we are watching. For case in point, our application may possibly have to have that we send an API request with the preliminary details and then repeat the process if the info changes.

We can power a watcher to be executed straight away by working with the handler purpose like we did in the preceding illustration, and also established its instant alternative to accurate:

By performing this, we can have our watcher get a selected motion as quickly as our app is launched and carry on to do so when the property we are observing modifications in the upcoming.

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With Vue 3, the instant solution can also be included to an optional item, as revealed under:

Also, the Composition API introduces a new watchEffect() strategy that is pretty related to observe() with the instant possibility set to real. Even so, whilst the look at operate/choice just tracks the explicitly monitored supply, watchEffect() will quickly track each and every reactive property accessed throughout its execution:

In this way, the preliminary price of foo and bar is logged to the console, and will go on to be logged whenever their value modifications.

Case in point eventualities of watchers in motion

To further more understand how watchers operate in a true life software, let us go around some simple illustrations.

Observe typing state

A uncomplicated and intuitive experiment with watchers is an software that watches a typing state and performs certain actions as the person is typing:

As in this illustration, we established two reactive states (information and typing). We then bind the concept to an enter ingredient and also produce a watcher for it, so that whenever it modifications, we established the typing worth from false to legitimate and quickly set it back to fake soon after 2 seconds.

When we operate this instance in the browser, we’ll get the pursuing consequence:

Hello World

Though this instance is fundamental, you could consider it a move further by sending a ask for to a distant API and updating your markup centered on the response from this ask for, comparable to how research engines like Google provide lookup tips as you type.

Real-time converter

One more interactive example is a simple converter application, where the output can be influenced and calculated centered on distinctive input details.

Like with the code under, we established a temperature converter, exactly where the Fahrenheit price could be calculated by getting into a Celsius price and vice versa:

Managing this would also give us the output below:

Temperature Converter

Simple countdown timer

To see the watchEffect() function in motion, we have also produced a simple countdown timer software below, which will start counting from 10 seconds following our software is mounted and will stop when the timer reaches 5 seconds:

The logic at the rear of this is really straightforward. We’d made a reactive counter property with a value of 10. We then created a watchEffect() that will continuously lower our counter by 1 each individual next, and halt anytime its benefit is at 5.

Halting a watcher

When the mum or dad part is unmounted, watchers prevent immediately, and in most instances, you will not need to have to finish a watcher manually. Nevertheless, there are scenarios in which you’d want to halt a watcher, perhaps when a situation has been met, and it’s really simple to do so.

With the Composition API, we’ll only have to have to assign our watcher to a personalized variable and then invoke this variable to stop the watcher, as witnessed below:

const unwatch = observe(foo, (value) => 
  // ...

// end watcher

The similar would function with watchEffect() much too. Even so, with the Options API, we can only halt a watcher programmatically if it has been produced with the this.$look at() process and not by using the watcher alternative:

In this example, we set up a new watcher in our app’s designed hook, and after 5 seconds, we stopped the watcher.

Computed houses vs. watchers

There is a whole lot of uncertainty about when to make the most of computed properties and when to use watchers. Notwithstanding, this segment can make clear the scenario.

Computed houses are made use of to compute the value of a residence centered on some other problems. Watchers, on the other hand, are not mainly employed for switching the worth of a property alternatively, they are utilized to notify you when the benefit has altered and enable you carry out specified steps primarily based on these adjustments.

Computed attributes need to be applied when you want to get the present benefit of an object and use it in your logic, these as calculating anything primarily based on it. And watchers ought to be employed when you have to have to know when just one or additional values have changed and react appropriately.


Watchers are a impressive element in Vue. They allow us to react promptly and dynamically to modifications in the procedure.

If a watcher is additional to an aspect, any time it adjustments, we can act on that alter. Although this may seem to be like an extraordinary case, it’s a excellent illustration of how being ready to react to variations can be valuable. And in this write-up, we were capable to go around in depth how to use watchers as well as its obtainable choices. In addition, we appeared at numerous sensible illustrations of how to use watchers in authentic-globe purposes.

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