Vue.js reactivity is a core feature that enables developers to create dynamic and responsive applications effortlessly. At its core, Vue.js employs a system of reactive components that automatically update the view whenever the underlying data changes. This is achieved through a reactive data model that leverages getter and setter functions, which track dependencies and trigger updates when necessary. This seamless synchronization between data and the user interface not only enhances performance but also simplifies the development process, making it easier for developers to manage state across their applications.

Understanding how reactive components work in Vue.js involves recognizing the reactivity system, which includes concepts like observables, watchers, and the virtual DOM. Each time a component's data changes, Vue.js efficiently processes the changes and updates the DOM only where required, rather than re-rendering the entire page. This ensures that applications remain fast and responsive, even as they scale. Additionally, using the computed properties and watch options allows developers to define how specific data should be derived or tracked, further enhancing the reactivity and efficiency of their applications.

When working with Vue.js, developers often encounter several common pitfalls that can hinder the performance and maintainability of their reactive components. One of the most frequent mistakes is the improper use of computed properties. Instead of leveraging computed properties for complex logic, many opt for methods that do not cache results, resulting in unnecessary recalculations during re-renders. To avoid this, always assess whether a piece of calculated state should be a computed property rather than a method. This not only improves performance but also enhances the readability of your code.

Another significant issue arises when modifying state directly. In Vue.js, state should only be modified through reactive data properties or methods to ensure proper tracking and reactivity. When developers mutate state directly, it can lead to unpredictable behavior and debugging difficulties. To prevent this, implement Vuex for centralized state management or utilize Vue's built-in methods such as this.$set or the spread operator for arrays and objects. By adhering to these guidelines, you ensure that your reactive components remain predictable and maintainable.

Vue.js is renowned for its reactivity, which is a core feature that sets it apart from other JavaScript frameworks. The **reactive system** of Vue.js is built upon its use of an observable data structure. When data is defined in a Vue instance, it is transformed into getters and setters using Object.defineProperty. This transformation allows Vue to track dependencies and automatically update the DOM whenever the data changes, ensuring that the user interface reflects the current state of the application.

Each **Vue component** acts as a reactive entity, meaning any change in a component's state triggers a cascading re-evaluation of its rendered output. This is achieved through the virtual DOM which Vue creates, effectively minimizing the direct manipulation of the actual DOM. Additionally, Vue’s reactivity system ensures efficient updates by employing a dependency tracking mechanism, where components that depend on specific data properties are only re-rendered when those properties change, enhancing performance and user experience.