How to Fix Uncaught TypeError: Cannot Read Properties of Undefined in React

Learn to solve the common error in React that occurs when reading properties of undefined. We'll break down the solution step by step with examples!
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How to Fix Uncaught TypeError: Cannot Read Properties of Undefined in React: A Complete Guide

When building applications with React, encountering errors like Uncaught TypeError: Cannot read properties of undefined (reading 'includes') can be frustrating, especially during the first render when working with arrays within objects. This issue often arises when you try to access properties of an object that is not fully defined or initialized.

In this guide, we'll troubleshoot and examine the causes of this error and provide you with a step-by-step guide to effectively resolve it, ensuring a smoother rendering of your components.

The Problem Explained

You have an array of sections, and each section contains a teachers array.

Understanding this problem will help us move forward with a solution.

Solution Overview

Step 1: Implement Null Check

The first change you can make is adding a null-check using optional chaining when filtering the sections. Here’s the updated code snippet:

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

In this code:

Step 2: Use useMemo for Derived State

While the previous step fixes the error, storing derived state using useState can be considered an anti-pattern in React. Instead, you can use the useMemo hook to optimize performance by recalculating availableSection only when necessary:

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

Here, useMemo ensures that the availableSection is calculated only if sections or teacherId changes, thus enhancing performance.

Step 3: Simplifying Selection Logic

If the primary goal of using local state is to compute the availableSection, you can consider moving the filtering logic directly into the useSelector hook:

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

This makes the data flow clearer and eliminates the need for additional useState or useEffect hooks.

Conclusion

By applying these strategies, you will effectively prevent the Uncaught TypeError: Cannot read properties of undefined error, streamline your React components, and ensure that the Sections are displayed correctly on the /dashboard page after the very first render. Remember, working with React often requires us to understand and manage the state carefully, particularly when dealing with asynchronous data fetching.

Don't forget to test your code regularly and ensure that you handle scenarios where your data might not be immediately available. Happy coding!