# 警告：useEffect 不能返回除函数以外的任何内容，该函数用于清理。您返回了：undefined

- **ID:** `react/useeffect-cleanup-return-non-function`
- **领域:** react
- **类别:** runtime_error
- **验证级别:** ai_generated
- **修复率:** 90%

## 根因

useEffect 回调返回了一个非函数值（例如来自异步函数的 undefined、Promise 或数字），而不是清理函数或什么都不返回。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| react@18.2.0 | active | — | — |
| react@16.14.0 | active | — | — |

## 解决方案

1. ```
   Define an async function inside the useEffect and call it without returning it. The useEffect callback itself should not be async.
   ```
2. ```
   If a cleanup is needed, return a function that performs the cleanup. If not, omit the return statement entirely.
   ```

## 无效尝试

- **Making the useEffect callback async directly (async () => { ... })** — An async function always returns a Promise, which is not a cleanup function, triggering the warning. (95% 失败率)
- **Adding a return statement that returns a string or boolean to suppress the warning** — Only a function (or nothing) is allowed; any other type will still cause the warning. (90% 失败率)
- **Wrapping the entire useEffect in a try-catch block** — The warning is about the return type of the callback, not about errors inside it; try-catch doesn't change the return value. (75% 失败率)