Javascript implementation of ruby gsub

gsub.js

String.prototype.gsub = function (pattern, replacement) {
    var match, result, source = this.toString();
    if (pattern == null || replacement == null) {
        return source;
    }
    result = '';
    while (match = source.match(pattern)) {
        result += source.slice(0, match.index);
        result += typeof replacement === 'function' ? replacement(match[0]) : replacement;
        source = source.slice(match.index + match[0].length);
    }
    return result + source;
};