Why can't I do array[-1] in JavaScript?

Question

In Python, you can do that:

arr = [1,2,3]
arr[-1] // evaluates to 3

But in JS, you can\'t:

let arr = [1,2,3];
arr[-1]; // e         


        

Answer 1

Because most languages like the indexOf function to return -1 instead of a needless exception. If -1 is a valid index then following code would result in 3 instead of undefined.

var arr = [1,2,3]
console.log(arr[arr.indexOf(4)])

IMHO, Python made a mistake by make negative indexes valid, because it leads to many strange consequences that are not directly intuitive.