// 计算bmi前检查是否已经输入了身高体重
if (height && weight) {
    var bmi = weight/(height*height);
    var status;
    if (bmi > 32) {
        status = "严重肥胖";
    } else if ( bmi >= 28) {
        status = "肥胖";
    } else if ( bmi >= 25) {
        status = "过重";
    } else if ( bmi >= 18.5) {
        status = "正常";
    } else {
        status = "过轻";
    }

    alert(`你的状态是${status}`);
} else {
    alert(`请检查是否已经输入了身高体重！`);
}