How to Create a Negative Index in JavaScript?

In JavaScript, you can create a negative index by simply using a negative number as the index to access elements in an array. For example:

let arr = [10, 20, 30, 40, 50];

console.log(arr[-1]);  // Output: 50
console.log(arr[-2]);  // Output: 40

Note that negative indices are relative to the end of the array, so arr[-1] returns the last element in the array (50), arr[-2] returns the second to last element (40), and so on.

Keep in mind that when using a negative index to access elements in an array, if the absolute value of the index is greater than the length of the array, it will return undefined.