format method
Implementation
String format(String amount) {
final chars = amount.split('');
String newString = '';
for (int i = chars.length - 1; i >= 0; i--) {
if ((chars.length - 1 - i) % 3 == 0 && i != chars.length - 1) newString = separator + newString;
newString = chars[i] + newString;
}
return currency.isEmpty ? newString : newString + ' ' + currency;
}