function getCategory(age) {        var category = "";        switch (true) {            case isNaN(age):                category = "not an age";                break;            case (age >= 50):                category = "Old";                break;            case (age <= 20):                category = "Baby";                break;            default:                category = "Young";                break;        };        return category;    }    getCategory(5);  // 将返回 "Baby"