调用\u validateParams时,可选属性的作用是什么?

当你打这种电话的时候

var e = Function._validateParams(arguments, [
                { name: "target", type: Array, elementMayBeNull: true },
                { name: "index", type: Number, integer: true },
                { name: "deserializing", type: Boolean, optional: true }
            ]);