Understanding Why useContext Returns Undefined Upon Redux Store Updates

Описание: Learn how to troubleshoot `useContext` returning undefined in React when the Redux store updates, and discover effective solutions to ensure a smooth functionality of your WebSocket connections.
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This video is based on the question https://stackoverflow.com/q/67797047/ asked by the user 'OmarSaid' ( https://stackoverflow.com/u/9139539/ ) and on the answer https://stackoverflow.com/a/67797150/ provided by the user 'UjinT34' ( https://stackoverflow.com/u/9602863/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Why useContext Returns Undefined Upon Redux Store Updates

In React development, using the Context API alongside Redux can sometimes lead to unexpected behaviors. A common issue is when useContext returns undefined after an update in the Redux store. If you've faced this problem, you're not alone! Let's explore this issue further and provide a clear explanation of how to fix it.

The Problem

Imagine you have a WebSocket connection set up in your React application using the Context API. You correctly expect to retrieve the WebSocket object through useContext in a child component. However, upon dispatching an action that updates the Redux store, you notice that the value returned by useContext turns undefined. This can be frustrating and lead to errors in your application, especially when you're trying to send messages or handle WebSocket events.

Example Scenario

You have a context that manages the WebSocket connection, and you are dispatching actions that update the Redux store. This ultimately leads to the WebSocket context (ws) becoming undefined, which disrupts your application flow. Here's a simplified version of your context setup:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

Understanding useSelector Behavior

The useSelector hook from Redux triggers a rerender whenever the selected value in the Redux state changes. In your context code, ws starts as undefined and is assigned a value only if the webSocket.current is (falsy). However, this logic results in ws being reset back to undefined when the component re-renders upon state updates.

Shifting the Scope of ws

To resolve this issue, you should consider moving the declaration of ws to a level where it does not get reset on each render. Here's how to do that:

Initialize ws outside the conditional statement that checks webSocket.current.

Make sure it retains its reference even after dispatches.

Here's an updated version of your context that implements this change:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By understanding how useSelector affects rerenders and adjusting where you define the WebSocket context value, you can avoid issues where useContext returns undefined. This small change enhances the reliability of your WebSocket functionality throughout your application.

If you run into any further issues or have additional questions, don’t hesitate to reach out to the community or dive deeper into the React and Redux documentation. Happy coding!