Format a numeric value with spaces between every third digit.

format_number.js

function formatNumber(value) {
  var input = value.toString(),
      inputParts = input.split('.'),
      int = inputParts[0],
      decimals = inputParts.length > 1 ? '.' + inputParts[1] : '',
      output = [];

  while (int.length > 3) {
    output.unshift(int.substr(-3));
    int = int.substr(0, int.length - 3);
  }
  if (int.length > 0) {
    output.unshift(int);
  }

  return output.join(' ') + decimals;
}