The “TypeError: Cannot read properties of undefined (reading ‘map’)” error occurs when you’re trying to call the `map()` function on an undefined value. This usually happens when your code assumes that a certain variable or property exists, but it doesn’t. In the context of React-JS, this error often occurs when working with arrays or objects that are not properly defined or initialized.
Before we dive into the solutions, let’s take a look at some common scenarios that can trigger the “TypeError: Cannot read properties of undefined (reading ‘map’)” error:
-
Using `map()` on an undefined array or object:
const arr = undefined;
arr.map(item => console.log(item));
-
Trying to access a non-existent property in an object:
const obj = { foo: 'bar' };
obj.nonExistentProperty.map(item => console.log(item));
-
Using `map()` on an array that’s still being fetched or initialized:
const [data, setData] = useState([]);
data.map(item => console.log(item));