A String.Format for JavaScript

The following is my implementation of a miniature version of .NET's String.Format:

String.format = String.format || function (format) {
    var params = [].splice.call(arguments, 1), nextOpen, nextClose = 0, index, plValue, spaces, totalSpaces;

    while ((nextOpen = format.indexOf('{', nextClose)) >= 0) {
        if (isNaN(+format[nextOpen+1])) {
            nextClose = nextOpen + 1;
            continue;
        }
        nextClose = format.indexOf('}', nextOpen);
        index = format.substring(nextOpen + 1, nextClose);
        if (index.indexOf(',') > 0) {
            spaces = +index.substring(index.indexOf(',') + 1);
            index = +index.substring(0, index.indexOf(','));
        }
        plValue = params[index] + '';
        if (spaces) {
            totalSpaces = new Array(Math.abs(spaces) - plValue.length + 1).join(" ");
            plValue = spaces > 0 ? totalSpaces + plValue : plValue + totalSpaces;
        }
        format = format.substring(0, nextOpen) + ((!plValue && plValue != 0) ? "" : plValue) + format.substring(nextClose + 1);
        spaces = 0;
    }
    return format;
};

Usage

String.format("Hello {0} {1}", "Andreas", "Grech");  // "Hello Andreas Grech"

for (var i = 0; i <= 1000; i += 200) {
    String.format("{0, 5}. Something", i);
}

/* Output:
    0. Something
  200. Something
  400. Something
  600. Something
  800. Something
 1000. Something
*/

String.format("Padded from {0, 7} left", "the");  // "Padded from     the left"

You can fork this project at github: https://github.com/dreasgrech/JSStringFormat