An AngularJS filter to format negative numbers with the currency filter.

negativeNumberFilter.js

app.filter('customCurrency', function ($filter) {       
  return function(amount, currencySymbol){
     var currency = $filter('currency');         

     if(amount.charAt(0) === "-"){
        return currency(amount, currencySymbol)
          .replace("(", "-")
          .replace(")", ""); 
     }

     return currency(amount, currencySymbol);
  };
});