Скачать или смотреть Solving undefined Values in useSelector with React/Redux and TypeScript

Discover how to resolve the common issue of receiving `undefined` values when using the `useSelector` hook in your React/Redux application with TypeScript.
---
This video is based on the question https://stackoverflow.com/q/63222573/ asked by the user 'AnroyPaul' ( https://stackoverflow.com/u/5457619/ ) and on the answer https://stackoverflow.com/a/63222917/ provided by the user 'tmhao2005' ( https://stackoverflow.com/u/3104226/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: useSelector hook returns 'undefined' from initial state

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Tackling the undefined Issue in useSelector for React/Redux

When developing applications with React, Redux, and TypeScript, it's not uncommon to face a few challenges along the way. One such challenge arises when trying to retrieve data from the Redux store using the useSelector hook, particularly when the returned values are undefined. If you're experiencing this yet have laid out your initial state properly, you're not alone. Let's dig into why this happens and how to fix it.

Understanding the Problem

In the given scenario, the developer has set up a simple test application, yet when trying to access values from the initial Redux state, they encounter undefined values. Here’s what the developer has already implemented:

Defined an interface for CurrencyExchangeState that contains fields like sum, mode, and currentCurrency.

Created an initial state that correctly initializes these fields.

Designed a Redux store using combineReducers where the currency reducer has been combined successfully.

However, when the developer attempts to select the currentCurrency value using the useSelector hook, the result returns undefined. This issue points to a common oversight when combining reducers in Redux.

The Solution

The root cause of receiving undefined can often be traced back to how the reducer’s state is accessed after using combineReducers. Let’s clarify and guide you through the solution step-by-step.

Step 1: Understanding combineReducers

When using combineReducers, Redux wraps the individual reducers you create into one root reducer. Every report on state now expects a slightly different structure that includes the names of the combined reducers.

Step 2: Adjust Your Selector

Given that you have combined your reducers, you need to adjust the useSelector hook to look for currentCurrency within the proper structure that your combined reducer provides. Instead of directly accessing state.currentCurrency, you should access it through the name you used during the combining process. Here’s the corrected selector:

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

Step 3: Update Your Component

With the updated selector, your component should look like this:

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

Important Points to Remember

Reducer Structure: Always ensure the structure of your state closely follows the naming convention used in combineReducers.

Strong Typing with TypeScript: Make good use of TypeScript types, like RootState, to prevent potential errors and promote better code practices.

Conclusion

By understanding the relationship between your combined reducers and the state structure, you can effectively retrieve data from your redux store. Adjusting the selector to reference the specific part of the state that corresponds with your reducer's name will resolve the undefined issue you were facing. Keep experimenting with your code, and you'll find that these concepts will soon become second nature.

If you have further issues or questions regarding React, Redux, and TypeScript integration, feel free to reach out! Happy coding!