Sometimes, you work with Vue.js and everything runs smoothly, but suddenly you realize you need to send data to the backend. This is where either Computed Properties or Watchers come into play. Sometimes it’s confusing to decide which one to use, right? I’ve also faced this dilemma many times in my projects. Sometimes you choose one, then realize the other might be more appropriate. Anyway, let’s take a closer look at Computed Properties and Watchers.
Let’s start with Computed Properties. As the name suggests, these are calculated properties. When you need to compute a value based on other data, Computed Properties come in handy. For example, suppose you have a product list and want to calculate the total price. Or, combine first name and last name to create a full name. These are perfect cases for Computed Properties.
The best part of Computed Properties is their caching feature. That is, if you perform a calculation and the underlying data hasn’t changed, Vue will retrieve the value from cache instead of recalculating. This significantly improves performance, especially for complex calculations. Imagine clicking a button that triggers many operations; this is where Computed Properties prove their utility. Of course, they re-compute whenever a dependent data changes, which makes sense.
Now, let’s talk about Watchers. As the name indicates, these observe a specific data property. When that data changes, they run a certain function. This is particularly useful for asynchronous operations, API calls, or complex DOM updates based on data changes. For example, when a user types in a search box, you might want to send a search query immediately. In such cases, Watchers are an excellent choice.
Unlike Computed Properties, Watchers do not cache results. When observed data changes, the watcher function runs every time. This can lead to unnecessary API calls or processing if not handled carefully. Therefore, you should prefer Watchers when you need to monitor when a data property changes and respond to it. If you just want to compute a value and use caching, Computed Properties are more suitable in my opinion.
Now, let’s look at some practical scenarios. Suppose in a Vue.js app, you’ll send user-inputted text to the backend. You might keep this text in a data property and send it when a button is clicked. This is a simple way to handle it.
But what if you want something more dynamic? For example, as the user types, you want real-time responses—like warnings when certain length is reached or errors if the format is incorrect. Here, Watchers come into play. You can observe the input text and perform checks on every change. While you could also do this with a Computed Property, recalculating on every keystroke might be inefficient. Watchers are more flexible for this.
Let’s consider another example: you have a user list with name, surname, and age, and you want to filter users over a certain age. Doing this with a Computed Property is sensible, because the filtered list remains the same unless the age filter changes. This improves performance by caching.
Regarding the “Wrong vs. Correct” approach, I can give an example. Suppose you’re creating a user’s full name. Here’s a suboptimal way:
// WRONG APPROACH data: { firstName: 'Ali', lastName: 'Veli' }, methods: { getFullName() { return this.firstName + ' ' + this.lastName; } } // Usage: {{ getFullName() }}This method is called and recalculated on every DOM update or method call, potentially hurting performance if it’s complex or called often. The better way is:
// CORRECT APPROACH (Computed Property) data: { firstName: 'Ali', lastName: 'Veli' }, computed: { fullName() { return this.firstName + ' ' + this.lastName; } } // Usage: {{ fullName }}As you see, `fullName` is a computed property. If `firstName` or `lastName` doesn’t change, the cached value is used, making it more efficient. For such calculations, computed properties are highly effective.
Now, a watcher scenario: suppose there’s a search box, and as the user types, you want to make an API request automatically. Here’s an example:
// Watcher Example data: { searchTerm: '' }, watch: { searchTerm(newValue, oldValue) { console.log('Search term changed:', oldValue, '->', newValue); // Call API here this.fetchResults(newValue); } }, methods: { fetchResults(query) { // Example API call console.log('API called: ', query); // fetch('/api/search?q=' + query)... } } // Usage: <input type="text" v-model="searchTerm">In this code, when `searchTerm` changes, the watcher runs, receiving the new value and triggering an API request. This real-time response is what users often see in search suggestions or live results.
In conclusion, understanding when to use Computed Properties versus Watchers helps you write more efficient and maintainable code. Use Computed Properties for derived, cacheable calculations based on reactive data. Use Watchers to respond to data changes with side effects like API calls or DOM updates. Sometimes, combining both is beneficial for complex scenarios. You can even watch a computed property for additional reactions, offering flexibility. You can find more about this in Vue.js official documentation or tutorials like this YouTube search.
Ultimately, both are core parts of Vue.js reactivity system. Knowing when and how to use each makes you a better developer. In my own projects, I often use them to simplify and optimize logic. For example, in a project, I needed to calculate form completion percentage, which was perfect for a computed property. Sometimes, simple solutions make the biggest difference, and these features do just that. I’ve tested them myself and found them very effective.