Can I decrease an array index by 1 and loop back to the end in a single line?

Question

Lets say that I'm manually adjusting the index of an array in JavaScript. I have a function that will increase the index and another that will decrease it. I want these functions to loop (so increasing the index when already at the end will reset the index to 0 and decreasing when the index is at 0 will reset the index to the array length - 1).

I can create the increase function in a single line. Is there a way to write the decrease function in a single line as well?

const array = [ /* ... */ ];
let index = 0;

function increase() {
    // One line - this makes me happy for some reason.
    index = (index + 1) % array.length;
}

function decrease() {
    // More than one line - this makes me irrationally sad.
    index -= 1;
    if (index < 0) {
        index = array.length - 1;
    }
}

I know it's possible to one-line the decrease function by using a ternary, but I feel like that's a bit of a cheat (I also find that multi-lining ternaries makes them much easier to read).

function decrease() {
    index = (
        index < 1
        ? array.length
        : index
    ) - 1;
}

Answer 1

You may try this line :

function decrease() {
    index = (index - 1 + array.length) % array.length;
}

You decrease the index by 1, and add the length of the array to transform a negative index in a positive one. Then you apply your modulo to stay in the boundaries of the array.