Explain the Vue.js reactivity system.
Instruction: Describe how Vue.js achieves reactivity and the principles behind it.
Context: This question assesses the candidate's understanding of one of the core features of Vue.js - its reactivity system and how it tracks changes to data.
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Vue.js's reactivity system is designed to automatically track and reflect changes in the application's state (data) onto the UI, ensuring that what the user sees is always up-to-date with the underlying data model. At the heart of this system are Vue's reactive objects. When you define your component's data properties, Vue wraps these properties in getter/setter pairs using Object.defineProperty. This process is what fundamentally enables the automatic tracking of changes. The system works in a beautifully streamlined manner...