Unexpected behavior

[Bruception]

Cases:

K is negative:

const quickselect = require('quickselect');

const numbers = [1, 2, 3, 4];
quickselect(numbers, -99999);

console.log(numbers); // [ 3, 4, 2, undefined, '-99999': 1 ]

K is greater than numbers.length:

const quickselect = require('quickselect');

const numbers = [1, 2, 3, 4];

quickselect(numbers, 99999);
console.log(numbers); // [ 3, 4, undefined, 1, <99995 empty items>, 2 ]

[pygy]

I'd consider this a documentation issue:

The array should have at least one element. The bounds and k are non-negative integers smaller than array's length. left should be smaller than or equal to right, and k must be in range. Proceeding otherwise leads to undefined behavior.

The calling code is in control, and can check for out of bounds errors if it doesn't directly control what's going in.

Edit... I was about to submit a PR, but in context, that language feels somewhat user hostile... Ideally you'd have a devMode that checks for valid input.