> For the complete documentation index, see [llms.txt](https://shenjunhong.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://shenjunhong.gitbook.io/blog/yuan-ma/react/errorboundaries.md).

# errorBoundaries

##

错误边界就是 一个生命周期，用来监听当前组件的children渲染过程，并可以返回一个降级的UI来渲染。

<https://live.bytedance.com/9788/9071260>

```js
class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    // 更新 state 使下一次渲染能够显示降级后的 UI
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    // 我们可以将错误日志上报给服务器
    logErrorToMyService(error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      // 我们可以自定义降级后的 UI 并渲染
      return <h1>Something went wrong.</h1>;
    }

    return this.props.children; 
  }
}


```

render => 任务调度 => 任务循环 => 提交 => 展示

抛出的异常会进入到我们的handleError 流程此时我们处理的目标。

```js
function handleError(root, thrownValue): void {
  let erroredWork = workInProgress; // 当前处理的 FiberNode 也就是异常的 节点
  throwException(
    root, // 我们的根 FiberNode
    erroredWork.return, // 父节点
    erroredWork,
    thrownValue, // 异常内容
  );
    completeUnitOfWork(erroredWork);
}

function throwException(
  root: FiberRoot,
  returnFiber: Fiber,
  sourceFiber: Fiber,
  value: mixed,
) {
  // The source fiber did not complete.
  sourceFiber.flags |= Incomplete;

  let workInProgress = returnFiber;
  do {
    switch (workInProgress.tag) {
      case HostRoot: {
        workInProgress.flags |= ShouldCapture;
        return;
      }
      case ClassComponent:
        // Capture and retry
        const ctor = workInProgress.type;
        const instance = workInProgress.stateNode;
        if (
          (workInProgress.flags & DidCapture) === NoFlags &&
          (typeof ctor.getDerivedStateFromError === 'function' ||
            (instance !== null &&
              typeof instance.componentDidCatch === 'function' &&
              !isAlreadyFailedLegacyErrorBoundary(instance)))
        ) {
          workInProgress.flags |= ShouldCapture;
          return;
        }
        break;
      default:
        break;
    }
    workInProgress = workInProgress.return;
  } while (workInProgress !== null);
}


```

throwException

将当前也就是出问题的节点状态标志为未完成 FiberNode.flags = Incomplete 从父节点开始冒泡，向上寻找有能力处理异常（ ClassComponent ）且的确处理了异常的（声明了 getDerivedStateFromError 或 componentDidCatch 生命周期）节点，如果有，则将那个节点标志为待捕获 workInProgress.flags |= ShouldCapture ，如果没有则是根节点。 completeUnitOfWork 方法也类似，从父节点开始冒泡，找到 ShouldCapture 标记的节点，如果有就标记为已捕获 DidCapture ，如果没找到，则一路把所有的节点都标记为 Incomplete 直到根节点，并把 workInProgress 指向当前捕获的节点。

之后从当前捕获的节点（也有可能没捕获是根节点）开始重新走流程，由于其状态 react 只会渲染其降级 UI，如果有 sibling 节点则会继续走下面的流程。我们看看上述例子最终得到的 FiberNode 树：


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://shenjunhong.gitbook.io/blog/yuan-ma/react/errorboundaries.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.