Separate number with JavaScript

separate-number.js

/* DISCLAIMER */
// This is all useless. Use Intl.NumberFormat instead.

const insertString = (string, index, newString) => {
  return string.substr(0, index) + newString + string.substr(index);
};

const separateNumber = (number, separator = " ") => {
  let string = String(number);

  for (let i = string.length - 1, j = 1; i > 0; --i, ++j) {
    if (j % 3 === 0) {
      string = insertString(string, i, separator);
      j = 0;
    }
  }

  return string;
};

Use Intl.NumberFormat if your browser supports it. This provides localisation including decimal places.