JavaScript: StringBuilder

/*****************************************************************
 * Stringbuilder Class - emulates the C#/VB.NET stringbuilder
 * @constructor
 *****************************************************************/
function StringBuilder() {
    var strings = [];
 
    this.append = function (string)
    {
        string = verify(string);
        if (string.length > 0) {
            strings[strings.length] = string;
        }
    };
 
    this.appendLine = function (string)
    {
        string = verify(string);
        if (this.isEmpty())
        {
            if (string.length > 0) {
                strings[strings.length] = string;
            }
            else {
                return;
            }
        }
        else {
            strings[strings.length] = string.length > 0 ? "\r\n" + string : "\r\n";
        }
    };
 
    this.clear = function () { strings = []; };
 
    this.isEmpty = function () { return strings.length == 0; };
 
    this.toString = function () { return strings.join(""); };
 
    var verify = function (string)
    {
        if (!defined(string)) {
            return "";
        }
        if (getType(string) != getType('string')) {
            return String(string);
        }
        return string;
    };
 
    var defined = function (el)
    {
        return el != null && typeof(el) != "undefined";
    };
 
    var getType = function (instance)
    {
        if (!defined(instance.constructor)) {
            throw new Error("Unexpected object type");
        }
        var type = String(instance.constructor).match(/function\s+(\w+)/);
 
        return defined(type) ? type[1] : "undefined";
    };
}