function FactorialFunction() {
    this.name = "FACTORIAL";
    this.maxArgs = 1;
    this.minArgs = 1;
}
FactorialFunction.prototype = new GC.Spread.CalcEngine.Functions.Function();
FactorialFunction.prototype.evaluate = function () {
    var result = 1, args = arguments;
    if (args.length === 1 && !isNaN(parseInt(args[0]))) {
        for (var i = 1; i < args[0]; i++) {
            result = i * result;
        }
        return result;
    }
    return "#VALUE!";
}

var factorial = new FactorialFunction();
activeSheet.addCustomFunction(factorial);
activeSheet.getCell(0,0).formula("factorial(5)");
var name = activeSheet.getCustomFunction("factorial");
alert(name);