Cannot read property of undefined React

Why This Happens

In React, you tried to access a property on undefined, usually before data loads. This is one of the most common React errors developers encounter. Understanding the root cause helps you fix it quickly and prevent it from recurring.

The Problem

function List({ items }) {
  return items.map(i => <li>{i}</li>);
}

The Fix

function List({ items }) {
  return items?.map(i => <li>{i}</li>);
}

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: Cannot read properties of undefined (reading 'map'). This tells you exactly what went wrong.

2. 2

   Find the cause

   Check the stack trace to find which line of your React code triggered this error.

3. 3

   Apply the fix

   Use the corrected code pattern shown above. Test to confirm the error is resolved.