Negative modulus test

How to handle modulus when the number goes negative. Although you don't need to if you're only subtracting 1 but this handles subtracting any amount.

by Terry King

HTML

<button id="prev">Prev</button>
<button id="next">Next</button>
<br>Output=<span id="output"></span>

JavaScript

vi = -1
$("#next").click(function () {
    vi = (vi + 1) % 7
    $("#output").text(vi)
});
$("#prev").click(function () {
    vi = (((vi - 1) % 7) + 7) % 7
    $("#output").text(vi)
});
$("#next").click();