Typeerror cannot read property ‘map’ of undefined

Encountering the “typeerror cannot read property ‘map’ of undefined” error message is quite normal when working with JavaScript.

If you’re having a hard time dealing with this “cannot read properties of undefined reading map,” don’t worry!

This is because we are going to hand you the solutions to this error.

Keep reading to understand what this error means and why it occurs in your code.

It is good to have a better understanding before troubleshooting the “cannot read property map of undefined” error message.

What is “typeerror cannot read property ‘map’ of undefined”?

The “typeerror: cannot read property ‘map’ of undefined” is a common error in JavaScript that occurs when you’re trying to use the map() method on an undefined or null value.

In addition to that, this error message indicates that the code is attempting to access the map() method on a variable that is not an array or has not been initialized with data.

For instance:

We try to use the map() method on this variable.

let sampleList;
console.log(sampleList.map(item => item.name));

As a result, it will throw:

TypeError: Cannot read properties of undefined (reading 'map')

The reason is that the map() method cannot be used on an undefined value.

What is map() method?

The map() method is used to create a new array by calling a provided function on every element in the calling array.

However, if the variable you’re trying to use the map() method on is undefined, it means that it doesn’t have any properties or methods.

What are the causes of “typeerror cannot read property ‘map’ of undefined”?

This error message occurs for several reasons, such as:

• Uninitialized variables
• Empty arrays
• Incorrect function parameters
• Typos in property names
• Asynchronous code

How to fix “typeerror cannot read property ‘map’ of undefined”?

To fix this error, you have to ensure that the variable you’re trying to use with the map() method is defined and is an array.

You can simply do it by verifying if the variable is undefined before using it or by initializing it with an empty array if it’s not already defined.

The following are the solutions you may use to resolve this error message.

Solution1: Check if the array exists before using the map() method

You need to check first if the myArray variable is undefined using an if statement.
If it’s not undefined, you can use the map() method to extract the name property of each object in the array.

For example:

const myArray = undefined;

if (myArray) {
  console.log(myArray.map(item => item.name));
} else {
  console.log("myArray is undefined.");
}

Solution 2: Set the initial state of the array to an empty array

Initialize the myArray state to an empty array using the useState() hook.

This ensures that the myArray variable is always an array and can be safely used with the map() method.

For example:

const [myArray, setMyArray] = useState([]);

useEffect(() => {
  // Code that sets the myArray state
}, []);

console.log(myArray.map(item => item.name));

Solution 3: Use a ternary operator to conditionally render the map() method

In this solution, you can use a ternary operator to conditionally render the map() method.

If myArray exists, we map over it and render a for each item.

If it’s undefined, an error message will be raised.

For example:

const myArray = undefined;

return (
  <div>
    {myArray ? myArray.map(item => <div key={item.id}>{item.name}</div>) : "myArray is undefined."}
  </div>
);

Solution 4: Use optional chaining

You can also use the optional chaining (?.) operator to check if the myArray variable is undefined before using the map() method.

If it’s undefined, the expression returns undefined instead of causing an error.

For example:

const myArray = undefined;

console.log(myArray?.map(item => item.name));

Take note!!!

The “typeerror: cannot read properties of undefined (reading ‘map’)” and “typeerror cannot read property ‘map’ of undefined” are essentially the same error message.

They both indicate that the map() method is being called on an undefined object, resulting in the error.

The distinction between these two errors is in the wording because of a recent change in the V8 JavaScript engine that powers Chrome and Node.js.

The error message was changed from “cannot read property ‘map’ of undefined” to “cannot read properties of undefined (reading ‘map’)” to make it more clear and consistent with other error messages.

Frequently Asked Questions (FAQs)

Conclusion

By executing all the effective solutions for the “typeerror cannot read property ‘map’ of undefined” that this article has already provided above, it will help you resolve the error.

We are hoping that this article provides you with sufficient solutions.

You could also check out other “typeerror” articles that may help you in the future if you encounter them.

• Typeerror cannot set properties of undefined
• Typeerror: ‘column’ object is not callable
• Typeerror [clientmissingintents]: valid intents must be provided for the client.

Thank you very much for reading to the end of this article.