ISO (SI) & APA style number formatting are used here to make a fail-safe, unambiguous way to render numbers for international and multicultural contexts.

formatISOAPA.js

// Number.formatISOAPA returns a correctly SI+APA formatted number.
// (SI, System Internationale) ISO 31-0 & APA style EXPLANATION:
// <https://en.wikipedia.org/wiki/ISO_31-0#Numbers>
// ISO 31-0: The decimal sign is either the comma on the line 
//           or the point on the line. Both "1,33" & "1.33" are valid.
// ISO 31-0: Do not use "." nor "," for thousand separators.
//           Both "1,000.00" "1.000,00" are INVALID.
// ISO 31-0: For numbers whose magnitude is less than 1, the decimal
//           sign should be preceded by a zero. "0.5" instead of ".5"
// APA style: Psychologically, there is no need to group numbers with 
//           just 4 figures (i.e. are less than "10 000.00").
//           "9999.99" and "2016" instead of "9 999.99" or "2 016".
// Usage:
// var num = 12345.6;
// English system: `num.formatISOAPA(2, "english");`
//                  returns => "12 345.60"
// French  system: `num.formatISOAPA(2, "french");` 
//                  returns => "12 345,60"
// English with ruby-like thousands separator: 
//                 `num.formatISOAPA(2, ".", "_");`
//                  returns => "12_345.60"

Number.prototype.formatISOAPA = function(decimalPrecision,
                                         decimalMark,
                                         thousandsSeparator) {
  "use strict";
  // Sanitize input.
  var n = (this + "").replace(/[^0-9+Ee.]/g, "");
  // The dash or the minus sign is the default negative mark.  
  var neg;
  if (this < 0) {
    neg = "-";
  } else {
    neg = "";
  }

  n = (!isFinite(+n)) ? 0 : +n;
  
  // Zero is the default decimal precision.
  var pre; 
  if (!isFinite(+decimalPrecision)) {
    pre =  0;
  } else {
    pre = Math.abs(decimalPrecision); 
  }

  // The dot is the default decimal mark. Comma is OK.
  var dec;
  // User can enter "," as the desired decimalMark, but also:
  // "FRENCH", "es_sp", "comma", "coma", or alternatively:
  // "English standard", "en", "dot", "point" or "pt".
  var decMarkCommaPatt = /^es|^fr|^com|\,/i;
  var decMarkDotPatt  = /^en|^do?t|^p\w*t|\./i;
  if ((typeof decimalMark === "undefined") || 
       (decMarkDotPatt.test(decimalMark))) {
    dec = ".";
  } else if (decMarkCommaPatt.test(decimalMark)) {
    dec = ",";
  } else {
    dec = decimalMark;
  }

  // The space is the default thousands separator. 
  // Underscores are OK. For HTML output, &thinsp; is even better!
  var ths = (typeof thousandsSeparator === "undefined") ? " " :
              thousandsSeparator;

  var parts = "";

  parts = (pre ? n.toFixed(pre) : "" + Math.round(n)).split(".");

  if (parts[0].length > 4) {
    parts[0] = parts[0].replace(/\B(?=(?:\d{3})+(?!\d))/g, ths);
  }
  if ((parts[1] || "").length < pre) {
    parts[1] = parts[1] || "";
    parts[1] += new Array(pre - parts[1].length + 1).join("0");
  }

  return neg + parts.join(dec);
 };