formatGroup.js

export default function(grouping, thousands) {
  return function(value, width) {
    var i = value.length,
        t = [],
        j = 0,
        g = grouping[0],
        length = 0;

    while (i > 0 && g > 0) {
      if (length + g + 1 > width) g = Math.max(1, width - length);
      t.push(value.substring(i -= g, i + g));
      if ((length += g + 1) > width) break;
      g = grouping[j = (j + 1) % grouping.length];
    }

    return t.reverse().join(thousands);
  };
}