**Course Title:** Mastering Vue.js: Building Modern Web Applications **Section Title:** Vue Directives and Event Handling **Topic:** Understanding computed properties and watchers. **Introduction** In the previous topics, we explored the fundamentals of Vue.js and its ecosystem, created our first Vue application, and learned about components, directives, and event handling. In this topic, we'll dive into two essential concepts in Vue: computed properties and watchers. These features enable us to write more efficient and reactive code, making our applications more responsive and user-friendly. **Computed Properties** Computed properties are functions that return a computed value based on reactive dependencies. They're cached until their dependencies change, making them more efficient than regular methods. **Why use computed properties?** 1. **Caching**: Computed properties cache their results, so we don't need to recompute them on every render, improving performance. 2. **Avoid unnecessary computation**: Computed properties only recompute when their dependencies change, reducing unnecessary computations. 3. **Easier code management**: By encapsulating complex logic in computed properties, we can simplify our templates and make our code more readable. **Example** Let's create a simple example that demonstrates the usage of computed properties. We'll create a Vue application that calculates the price of an item based on its quantity and tax rate. ```html <template> <div> <input type="number" v-model="quantity"> <input type="number" v-model="taxRate"> <p>Total price: {{ totalPrice }}</p> </div> </template> <script> export default { data() { return { quantity: 1, taxRate: 8 // in percent } }, computed: { totalPrice() { return this.quantity * this.taxRate / 100; } } } </script> ``` In this example, the `totalPrice` computed property depends on the `quantity` and `taxRate` reactive data properties. When either of these properties changes, the `totalPrice` computed property will recompute and update its value. **Watchers** Watchers are functions that allow us to observe changes to a particular reactive property or a collection of properties. We can use watchers to perform side effects or execute additional logic when a property changes. **Why use watchers?** 1. **Perform side effects**: Watchers enable us to perform side effects, like making API calls or updating external resources, when a property changes. 2. **Decouple logic**: By moving complex logic to watchers, we can separate it from our computed properties and methods, making our code more modular and maintainable. **Example** Let's create an example that demonstrates the usage of watchers. We'll create a Vue application that updates a user's profile information when their email address changes. ```html <template> <div> <input type="email" v-model="email"> <p>User profile updated at: {{ updatedAt }}</p> </div> </template> <script> export default { data() { return { email: '', updatedAt: new Date() } }, watch: { email(newEmail) { // Simulate an API call to update the user's profile setTimeout(() => { this.updatedAt = new Date(); }, 1000); } } } </script> ``` In this example, the `email` watcher observes changes to the `email` reactive property. When the `email` property changes, the watcher performs a side effect, updating the `updatedAt` property. **Best Practices** 1. **Use computed properties for caching**: Whenever you need to perform complex computations, use computed properties to cache the results and improve performance. 2. **Use watchers for side effects**: When you need to perform side effects or execute additional logic when a property changes, use watchers to decouple the logic from your computed properties and methods. **Conclusion** In this topic, we explored the usage of computed properties and watchers in Vue. These features enable us to write more efficient, reactive, and maintainable code. By following the best practices outlined above, you'll be able to leverage these features effectively in your Vue applications. **What's Next** In the next topic, 'Best practices for managing DOM updates,' we'll discuss strategies for optimizing DOM updates and improving application performance. We'll cover topics like using batch updates, optimizing re-renders, and leveraging Vue's built-in optimization features. **Do you have any questions or need further clarification on this topic?** Please feel free to ask for help by replying to this topic. External References used: - [Vue Official Documentation](https://vuejs.org/v2/api/#Computed-Properties) - [Vue Official Documentation](https://vuejs.org/v2/api/#Watchers)