In Vue 3, why can't the `setup()` function directly use `this` to access the component instance? The reason lies in the design of the Composition API. The `setup()` function is executed before the component instance is fully created, at which point the `this` pointer (which refers to the component instance) has not yet been established. The Composition API aims to organize logic in a functional, component-based manner rather than relying on the context provided by `this`. As a result, direct access to `data`, `methods`, `props`, and other options via `this` is not possible within the `setup()` function. Alternative approaches: - Props and context (including `attrs`, `slots`, `emit`, etc.) are explicitly passed as parameters to `setup(props, context)`. - Reactive state must be created using `ref()` or `reactive()` and explicitly returned for template usage. - Methods should be defined within `setup()` and returned explicitly. For example: ```js import { ref } from 'vue'; export default { setup(props, { emit }) { const count = ref(0); const increment = () => { count.value++; emit('update', count.value); }; return { count, increment }; // Template can directly use count and increment } } ``` This design improves code readability, testability, and logical reusability by avoiding the maintenance challenges associated with the Options API's context ambiguity and fragmented logic.