Format integer number

formatNumber.js

function format (n) {
  var str = n.toString()
  for (var i = str.length - 3; i > 0; i -= 3) {
    str = str.substring(0, i) + ',' + str.substring(i)
  }
  return str
}

var arr = [10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000, 10000000000]
arr.forEach(function (n) {
  console.log(format(n))
})