Function Number Format Untuk Javascript

function number_format(user_input){
    var filtered_number = user_input.replace(/[^0-9]/gi, '');
    var length = filtered_number.length;
    var breakpoint = 1;
    var formated_number = '';
    for(i = 1; i <= length; i++){
        if(breakpoint > 3){
            breakpoint = 1;
            formated_number = ',' + formated_number;
        }
        var next_letter = i + 1;
        formated_number = filtered_number.substring(length - i, length - (i - 1)) + formated_number;
        breakpoint++;
    }
    return formated_number;
}