TypeError: Cannot read properties of undefined

Shivan M.

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.

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.

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 (?) to access the map function on the posts array.

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

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.

Resources

• Optional Chaining (?.) - JavaScript | MDN