Скачать или смотреть How to Fix TypeError: Cannot read properties of undefined (reading 'map') in ReactJS

Learn how to solve the common ReactJS error: `TypeError: Cannot read properties of undefined (reading 'map')`. This guide guides you through understanding and resolving the issue step-by-step.
---
This video is based on the question https://stackoverflow.com/q/69315727/ asked by the user 'kelly-ghost_busters' ( https://stackoverflow.com/u/14755000/ ) and on the answer https://stackoverflow.com/a/69316115/ provided by the user 'Sinan Yaman' ( https://stackoverflow.com/u/12358693/ ) 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: ReactJS TypeError: Cannot read properties of undefined (reading 'map')

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 TypeError in ReactJS: A Guide to Fixing Your map Function

If you’re experiencing the dreaded TypeError: Cannot read properties of undefined (reading 'map') error while working with React components, you're not alone. This frustrating issue often stems from attempting to use the map function on an undefined value—most commonly occurring while fetching data from an API. In this guide, we will explore why this error occurs and how to fix it effectively.

The Problem: What Causes the Error?

The map function is a method used in JavaScript arrays to iterate over elements and apply a function to each element. If the array is either not defined or is not in an expected format, you will encounter this error. Here’s a typical situation:

You have a parent component that fetches data from an API and passes that data down to a child component.

If the data being passed down is undefined at the time when the child component tries to render it using the map method, you will see the type error.

In our scenario, the user is experiencing this when dealing with two components, InputForm.js and ViewAll.js. Even though the child component displays retrieved values correctly, the parent component does not extract them properly, leading to the error.

Analyzing Your Code

Let’s take a closer look at the provided code snippets from InputForm.js and ViewAll.js to understand the source of the problem.

InputForm.js

In InputForm.js, the important part to note is how the input data is being stored and passed to the ViewAll component:

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

Here, inputData is intended to be an array of objects. However, if no data is received yet, and ViewAll attempts to render this undefined value, you're bound to encounter the error.

ViewAll.js

In ViewAll.js, the following lines cause the problem:

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

If inputs is undefined or falsy when this line is executed, the error occurs because the map function is not being called on a valid array.

The Solution: Implementing Conditional Rendering

To prevent encountering this error, you need to ensure that inputs is defined and is an array before calling the map method. Here’s how you can adjust your ViewAll.js component:

Modified ViewAll.js

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

Key Improvements:

Conditional Rendering: By incorporating {inputs && inputs.length > 0 ? (...) : (...)}, you ensure that the map function only executes when there's actual data to iterate over.

Fallback Message: Including a fallback message will enhance user experience by informing users when no data is available.

Conclusion

By implementing these changes, you should be able to avoid the TypeError: Cannot read properties of undefined (reading 'map') issue in your React application. Remember, effective error handling and robust checks for undefined values will help maintain a smooth user experience and boost your application's reliability.

Happy coding!