ComponentDidCatch

`componentDidCatch` is a lifecycle method in React JS that catches JavaScript errors anywhere in a component's child tree. Instead of crashing the whole app, it gives you a chance to handle errors gracefully and display a fallback UI.

This lifecycle method is called during the "commit" phase, which allows side effects. It's like a JavaScript catch {} block, but for components.

Here is its syntax:
```
componentDidCatch(error, info)
```
- `error` is the error that was thrown.
- `info` is an object with `componentStack` key. The property `componentStack` is a string showing component stack trace.

A simple use of componentDidCatch could look like this:
```jsx
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}

componentDidCatch(error, info) {
// Display fallback UI
this.setState({ hasError: true });
// You can also log the error to an error reporting service
logErrorToMyService(error, info);
}

render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}
```
In this code, if a child component throws an error, the `componentDidCatch` lifecycle method gets triggered and sets the state `hasError` to true, causing the component to re-render and display a fallback UI ("Something went wrong.") instead of the actual UI that has caused the error. It can also be used to log error info to an error reporting service.