Translation and Refinement: In Vue 3, the setup function does not support asynchronous definition. This design choice is rooted in the component initialization phase's architectural principles. During the creation of the component instance, the setup function must execute synchronously and return reactive data and methods, ensuring Vue can correctly build the reactive system and rendering context. If the setup is defined as an asynchronous function, its return value will be encapsulated as a Promise, leading to runtime errors as Vue cannot directly access the required properties and methods. Analysis: The design intent of the setup function is to serve as the unified entry point for component initialization, requiring it to return an object (or rendering function). Defining setup as an asynchronous function will encapsulate its return value as a Promise, conflicting with the core mechanism of Vue's reactive system. For scenarios requiring asynchronous operations (e.g., API calls), it is recommended to encapsulate the async logic within lifecycle hooks like onMounted, or handle async tasks within the setup function using async/await, while ensuring the setup function itself remains a synchronous function. Example: ```javascript import { onMounted, ref } from 'vue' export default { setup() { const data = ref(null) onMounted(async () => { const response = await fetch('/api/data') data.value = await response.json() }) return { data } } } ``` This example demonstrates how to encapsulate asynchronous logic within onMounted while adhering to the synchronous requirement of the setup function, enabling efficient asynchronous data retrieval.