loop in list.

loop-in-list.js

var list = ['a', 'b', 'c', 'd'];
var cur;

function plus() {
  cur = (cur + 1) % list.length
}

function minus() {
  cur = (cur - 1 + list.length) % list.length
}

cur = 0;

console.log(cur);
plus();
console.log(cur);
plus();
console.log(cur);
plus();
console.log(cur);
plus();
console.log(cur);

cur = 3;

console.log(cur);
minus();
console.log(cur);
minus();
console.log(cur);
minus();
console.log(cur);
minus();
console.log(cur);