Formats a number as a currency (ie $1,234.56). When no currency symbol is provided, '$' used.

Usage:

{{ number_expression | number[:fractionSize] }}

@NgFilter(name:'currency')
class CurrencyFilter {

 NumberFormat nf = new NumberFormat();

 CurrencyFilter() {
   nf.minimumFractionDigits = 2;
   nf.maximumFractionDigits = 2;
 }

 /**
  *  [value]: the value to format
  *
  *  [symbol]: Symbol to use.
  *
  *  [leading]: Symbol should be placed in front of the number
  */
 call(value, [symbol = r'$', leading = true]) {
   if (value is String) value = double.parse(value);
   if (!(value is num)) return value;
   if (value.isNaN) return '';
   var neg = value < 0;
   if (neg) value = -value;
   var before = neg ? '(' : '';
   var after = neg ? ')' : '';
   if (leading) {
     return '$before$symbol${nf.format(value)}$after';
   } else {
     return '$before${nf.format(value)}$symbol$after';
   }
 }
}