Javascript check if an index equals a number

Question

I'm currently trying to check if the current active index equals to a number in the following way:

8, 16, 24, 32, 40 and so on... If that's the case then the console should print out: Go down.

The described part above works fine, but I can't seem to get the opposite part working.

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Let's say my currentIndex equals to 12 then the next possible go down index would be 16 (Works fine) and the next possible index to go up would be 9 since its only showing 8 items each time. Now I need to check if the currentIndex equals to 8 + 1 / 16 + 1 / 24 + 1 / 32 + 1 / 40 + 1 ... So the console would print out Go up.

How could I create my else if to achieve this?

var shownItems = 8;
var currentIndex = 12;

// Check if the currentIndex equals 8 / 16 / 24 / 32 / 40 ...
if(currentIndex / shownItems % 1 === 0) {
    console.log("Go down");
}
// Check if the currentIndex equals 8 - 7 / 16 - 7 / 24 - 7 / 32 - 7 / 40 - 7
else if() {
    console.log("Go up");
}

Answer 1

Just check for:

 (currentIndex + 1) % shownItems === 0