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Скачать или смотреть How to Fix the Cannot read property 'map' of undefined Error in React JS

  • vlogize
  • 2025-04-17
  • 1
How to Fix the Cannot read property 'map' of undefined Error in React JS
Error:Cannot read property 'map' of undefined REACT JSjavascriptarraysreactjsdictionaryfilter
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Описание к видео How to Fix the Cannot read property 'map' of undefined Error in React JS

Learn how to resolve the common "Cannot read property 'map' of undefined" error in React JS, especially when dealing with item deletion in arrays. This guide provides tips and code snippets to enhance your error handling.
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This video is based on the question https://stackoverflow.com/q/68334397/ asked by the user 'Yashvardhan Jain' ( https://stackoverflow.com/u/14888887/ ) and on the answer https://stackoverflow.com/a/68334439/ provided by the user 'Viet' ( https://stackoverflow.com/u/6108390/ ) 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: Error:Cannot read property 'map' of undefined REACT JS

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.
---
Understanding the "Cannot Read Property 'map' of Undefined" Error in React JS

If you're working with React and arrays, you might have stumbled upon the dreaded error: Cannot read property 'map' of undefined. This error is particularly common when performing operations that modify an array, such as deleting an item. In this post, we'll dissect this error and provide a clear solution to ensure your application runs smoothly without interruptions.

The Problem

In a scenario where you maintain an array of items (like notes) in a React component's state, you might encounter this error when you attempt to delete an item. Here's an example situation:

You have a notes application that allows users to add and delete notes.

When trying to delete a note, the application throws the error: "cannot read property 'map' of undefined."

It happens because the function that aims to delete the note does not correctly return the updated array.

Here's an excerpt from the code where the problem likely lies:

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

In the code above, the handleDelete function fails to return a new state. Instead of providing the filtered notes array back to setNotes, it only attempts to filter the notes without returning anything, leading to the error when attempting to map through an undefined array in the render.

The Solution: Properly Updating State

To resolve this issue, you'll want to ensure that your state update function properly returns a new state. Let's break it down:

Step 1: Update the handleDelete Function

Modify the handleDelete function to use filter correctly — ensuring that you return a new array that excludes the note at the specified index. Here's how you can do it:

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

Step 2: Ensure State is Updated

With the change above:

prevItems.filter(...) will create a new array without the deleted note.

The setNotes function will receive this newly filtered array, thus updating the state properly.

Now, when your component attempts to map over notes, it will no longer render an undefined value.

Conclusion

In summary, if you ever see the Cannot read property 'map' of undefined error while working with React, particularly during array manipulations, check your state management logic. Make sure your state update functions return new values, especially when filtering or deleting items from an array.

By following the provided solution to correctly filter notes and update the state, you can avoid interruptions in your application. Happy coding!

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