Whilst in JS, assuming p0 is a number, then mutation isn't going to happen, for people coming in from other languages it's uncomfortable to see. Generally it's a bad habit to 'mutate' arguments as you're increasing the cognitive load on future coders to know whether the argument is a reference or not. For example, if the code is refactored and now p0 is given as an object or an array - if the coder niavely updates the code, they could easily start actually mutating the argument. Of course, good testing will guard against this, but it is a bad habit unless you want mutation outside of the function.

I agree that this isn't best practice, though it is a decent solution. For instance, I wouldn't want the arguments to be changed in the function body as it leads questions about accidental mutation (particularly if you come from another language). Of course, the argument names are bad, but as they are given by the Kata I can understand keeping them the same. However y to stand as number of years is a poor choice. With modern IDEs, there's very little reason to not use descriptive variable names. So if this was a professional codebase, I would want some changes before I would accept this.

But definitely a good solution in terms of solving the problem