Format a number with a separator.

numberFormat.js

const numberFormat = (number, separator = ',') => {
  const num = Number.isNaN(+number) ? 0 : +number;
  let x = 0;
  let fnum = [];

  const digits = (num + '').split('');
  const seg = digits.length / 3;

  while(x < seg){
    fnum[x] = digits.splice(-3).join('');
    x++;
  }
  return fnum.reverse().join(separator);
}

numberFormat(100,','); //  100
numberFormat(1000,' '); //  1 000