modulus.js

function modulus(xs) {
  let output = [];
  while (true) {
    if (xs.length) {
      let [a, b] = [xs.shift(), xs.shift()];
      if (b > a) output.push(a);
      else {
        let c = b;
        while (true) {
          let d = a - c;
          if (d === 0) {
            output.push(0);
            break;
          } else if (d < 0) {
            c -= b;
            output.push(a - c);
            break;
          }
          c += b;
        }
      }
    } else break;
  }
  return output;
}

console.log(modulus([8, 3, 6, 6, 4, 2, 3, 7]));