# Fix TypeError: Cannot Read Properties of Undefined in JS

> Resolve the TypeError Cannot read properties of undefined error in JavaScript using optional chaining or conditional checks to safely access object properties

**URL:** https://sentry.io/answers/typeerror-cannot-read-properties-of-undefined/

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## The Problem

In JavaScript, you might encounter the following error when trying to iterate over an array with the `.map` function:

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

This error is the same as the `Cannot read property 'map' of undefined` error. The wording was changed in response to a [bug report](https://bugs.chromium.org/p/v8/issues/detail?id=11365).

This class of error ("Cannot read properties of undefined") occurs when a function (or, more broadly, a property) is accessed on a value that is not assigned. This could mean that a variable initialization did not occur or that an expected object is not present and so is undefined.

## The Solution

If you encounter this error in JavaScript, it is important to take note of the stack trace to identify where the error occurred. The offending line of code will include access to a property of a value that is undefined.

In the following code example, we try to map over the values in an array that might be undefined at the time of iteration.

```js
let posts;

function iterateOverPosts() {
  posts.map((post) => console.log(post.title));
}

iterateOverPosts();
```

If `posts` has not yet been updated when the `iterateOverPosts` function runs, our code will throw the `TypeError: Cannot read properties of undefined (reading 'map')` error.

To resolve this issue, we have two options:

1. Use [optional chaining](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining) (`?`) to access the `map` function on the posts array.

```js
let posts;

function iterateOverPosts() {
  posts?.map((post) => console.log(post.title))
}
```

By using optional chaining to access the `map` function, we ensure that if `posts` is `null` or `undefined`, our expression will short-circuit and evaluate to undefined instead of throwing the error.

2. Use a conditional expression to check if `posts` is a valid value

```js
let posts;

function iterateOverPosts() {
  if (posts) {
    posts.map((post) => console.log(post.title))
  } else {
    // Handle the case where posts is null or undefined
  }
}
```

As an alternative to optional chaining, you can use an `if` statement. This allows you to properly handle the case where `posts` is `null` or `undefined`.

## Tracking JavaScript Errors in Production

While optional chaining and defensive coding help prevent TypeErrors during development, [JavaScript error tracking](https://sentry.io/for/javascript/) in production captures the unexpected cases. Production environments often expose edge cases that are difficult to reproduce locally—like race conditions, timing issues, or data inconsistencies from external APIs. Error monitoring tools automatically capture these TypeErrors with full context, including the variable state, user actions, and execution path, making it easier to identify why values are undefined and fix the root cause.

## Resources

- [Optional Chaining (?.) - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining)

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*Source: [sentry.io/answers/typeerror-cannot-read-properties-of-undefined/](https://sentry.io/answers/typeerror-cannot-read-properties-of-undefined/)*