digit grouper

/* Invoke the function by using, for example, function digit_grouper(123456789); */
/* parameters – Uncomment by removing "//" to enable custom values. */
// var digits_per_group = 3;
// var digit_separator = ",";

function digit_grouper(
    // The order of the second and third parameter can be swapped if desired, but input_number should be at the beginning.
    input_number, digits_per_group, digit_separator
    ) {
        // defaults
    if ( isNaN(input_number) ) return false; // return on invalid input
    if (!digit_separator) digit_separator=","; // separation character if none specified
    if (!digits_per_group || isNaN(input_number) ) digits_per_group=3; // digits per group if none specified
        // prepending empty strings before numbers to force variable type into "string" instead of "number"
    var input_length = (""+input_number).length; // memorize length of input number
    var output = "" + input_number; // initiating output variable
    var count; // defeats JSHint error; no functional difference.
    if (input_length > digits_per_group && Math.floor(digits_per_group) > 0) /* skip grouping digits if shorter than digits per group; prevent division by zero that would lead to infinite loop */ {
        // insert "+1" after the first "digits_per_group" above in order to not group four-digit numbers like YouTube occasionally did on their mobile web site.
        for (
            count=1; // start counter at 1 to prevent trailing comma in slice
            count-1 < Math.floor( (input_length-1)/digits_per_group ) && count < 1000; // repeat splicing as many times as digit groups will be created. Limit to 1000 cycles for "sanity", i.e. to safe-guard against an endless loop should one occur due to possible undiscovered bugs. A user is anyway very unlikely to specify a number that long. Should it actually be necessary, this restriction can be altered.
            count++ // count up for each pass
        ) {
            output = output.substring(0,input_length-(count*digits_per_group) ) + digit_separator + output.substring(input_length-(count*digits_per_group) ); // insert separator
        }
    }
    return output;
}