TypeError: Cannot read property 'length' of undefined

This error occurs when you try to access the length property of a variable that is undefined.

For example:

let arr;
console.log(arr.length); // TypeError: Cannot read property 'length' of undefined

In this case, the variable `arr` is undefined and does not have a length property. To fix this error, you can initialize the variable with an empty array or check if the variable is defined before accessing its length property.

let arr = [];
console.log(arr.length); // 0

if (arr) {
  console.log(arr.length); // 0
}