Add commas or spaces to group every three digits

Just split into two parts with '.' and format them individually.

function commafy( num ) {
    var str = num.toString().split('.');
    if (str[0].length >= 5) {
        str[0] = str[0].replace(/(\d)(?=(\d{3})+$)/g, '$1,');
    }
    if (str[1] && str[1].length >= 5) {
        str[1] = str[1].replace(/(\d{3})/g, '$1 ');
    }
    return str.join('.');
}

Simple as that:

var theNumber = 3500;
theNumber.toLocaleString();

Here are two concise ways I think maybe useful:

1. Number.prototype.toLocaleString

This method can convert a number to a string with a language-sensitive representation. It allows two parameters, which is locales & options. Those parameters may be a bit confusing, for more detail see that doc from MDN above.

In a word, you could simply use is as below:

console.log(
   Number(1234567890.12).toLocaleString()
)
// log -> "1,234,567,890.12"

If you see different with me that because we ignore both two parameters and it will return a string base on your operation system.

2. Use regex to match a string then replace to a new string.

   Why we consider this? The toLocaleString() is a bit confusing and not all browser supported, also toLocaleString() will round the decimal, so we can do it in another way.

// The steps we follow are:
// 1. Converts a number(integer) to a string.
// 2. Reverses the string.
// 3. Replace the reversed string to a new string with the Regex
// 4. Reverses the new string to get what we want.

// This method is use to reverse a string.
function reverseString(str) { 
    return str.split("").reverse().join("");  
}

/**
 * @param {string | number} 
 */
function groupDigital(num) {
  const emptyStr = '';
  const group_regex = /\d{3}/g;

  // delete extra comma by regex replace.
  const trimComma = str => str.replace(/^[,]+|[,]+$/g, emptyStr)


  const str = num + emptyStr;
  const [integer, decimal] = str.split('.')

  const conversed = reverseString(integer);

  const grouped = trimComma(reverseString(
    conversed.replace(/\d{3}/g, match => `${match},`)
  ));

  return !decimal ? grouped : `${grouped}.${decimal}`;
}


console.log(groupDigital(1234567890.1234)) // 1,234,567,890.1234
console.log(groupDigital(123456))  // 123,456
console.log(groupDigital("12.000000001"))  // 12.000000001

Easiest way:

1

var num = 1234567890,
result = num.toLocaleString() ;// result will equal to "1 234 567 890"

2

var num = 1234567.890,
result = num.toLocaleString() + num.toString().slice(num.toString().indexOf('.')) // will equal to 1 234 567.890

3

var num = 1234567.890123,
result = Number(num.toFixed(0)).toLocaleString() + '.' + Number(num.toString().slice(num.toString().indexOf('.')+1)).toLocaleString()
//will equal to 1 234 567.890 123

4

If you want ',' instead of ' ':

var num = 1234567.890123,
result = Number(num.toFixed(0)).toLocaleString().split(/\s/).join(',') + '.' + Number(num.toString().slice(num.toString().indexOf('.')+1)).toLocaleString()
//will equal to 1,234,567.890 123

If not working, set the parameter like: "toLocaleString('ru-RU')" parameter "en-EN", will split number by the ',' instead of ' '

All function used in my code are native JS functions. You'll find them in GOOGLE or in any JS Tutorial/Book

If you are happy with the integer part (I haven't looked at it closly), then:

function formatDecimal(n) {
  n = n.split('.');
  return commafy(n[0]) + '.' + n[1];
}

Of course you may want to do some testing of n first to make sure it's ok, but that's the logic of it.

Edit

Ooops! missed the bit about spaces! You can use the same regular exprssion as commafy except with spaces instead of commas, then reverse the result.

Here's a function based on vol7ron's and not using reverse:

function formatNum(n) {
  var n = ('' + n).split('.');
  var num = n[0];
  var dec = n[1];
  var r, s, t;

  if (num.length > 3) {
    s = num.length % 3;

    if (s) {
      t = num.substring(0,s);
      num = t + num.substring(s).replace(/(\d{3})/g, ",$1");
    } else {
      num = num.substring(s).replace(/(\d{3})/g, ",$1").substring(1);
    }
  }

  if (dec && dec.length > 3) {
    dec = dec.replace(/(\d{3})/g, "$1 ");
  }

  return num + (dec? '.' + dec : '');
}