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JSFiddle：演示

HTML

<select name="meal" id="meal" onChange="changecat(this.value);">
    <option value="" disabled selected>Select</option>
    <option value="A">A</option>
    <option value="B">B</option>
    <option value="C">C</option>
</select>
<select name="category" id="category">
    <option value="" disabled selected>Select</option>
</select>

JS

var mealsByCategory = {
    A: ["Soup", "Juice", "Tea", "Others"],
    B: ["Soup", "Juice", "Water", "Others"],
    C: ["Soup", "Juice", "Coffee", "Tea", "Others"]
}

    function changecat(value) {
        if (value.length == 0) document.getElementById("category").innerHTML = "<option></option>";
        else {
            var catOptions = "";
            for (categoryId in mealsByCategory[value]) {
                catOptions += "<option>" + mealsByCategory[value][categoryId] + "</option>";
            }
            document.getElementById("category").innerHTML = catOptions;
        }
    }

实际上，JavaScript 支持某种循环，即 for...in 循环。

for...in 循环只迭代可枚举的属性。从内置构造函数（如 Array 和 Object）创建的对象从 Object.prototype 和 String.prototype 继承了不可枚举的属性，例如 String 的 indexOf() 方法或 Object 的 toString() 方法。循环将遍历对象本身的所有可枚举属性以及对象从其构造函数的原型继承的属性（更接近原型链中对象的属性覆盖原型的属性）。

在每次迭代中，对象的一个​​属性被分配给变量名，这个循环一直持续到对象的所有属性都用完为止。

更多链接

您可以使用onchangeevent 并使用 switch 语句和从第一个下拉列表中选择的值，并根据它将选项附加到第二个列表：

    var A= ["Soup", "Juice", "Tea","Others"];
    var B= ["Soup","Juice","Water", "Others"];
    var C= ["Soup","Juice","Coffee", "Tea","Others"];

var changeCat = function changeCat(firstList) {
    var newSel = document.getElementById("category");
    //if you want to remove this default option use newSel.innerHTML=""
    newSel.innerHTML="<option value=\"\">Select</option>"; // to reset the second list everytime
    var opt;

      //test according to the selected value
      switch (firstList.options[firstList.selectedIndex].value) {
          case "A":
              for (var i=0; len=A.length, i<len; i++) {
                    opt = document.createElement("option");
                    opt.value = A[i];
                    opt.text = A[i];
                    newSel.appendChild(opt);
              }
              break;
          case "B":
              for (var i=0; len=B.length, i<len; i++) {
                    opt = document.createElement("option");
                    opt.value = B[i];
                    opt.text = B[i];
                    newSel.appendChild(opt);
              }
              break;
          case "C":
              for (var i=0; len=C.length, i<len; i++) {
                    opt = document.createElement("option");
                    opt.value = C[i];
                    opt.text = C[i];
                    newSel.appendChild(opt);
              }
              break;
      }

}

<select name="meal" id="meal" onchange="changeCat(this);">
     <option value="">Select</option>
     <option value="A">A</option>
     <option value="B">B</option>
     <option value="C">C</option>
</select>

<select name="category" id="category" size="5">
     <option value="">Select</option>
</select>

我使用size="5"第二个下拉菜单来查看每个选择的实时结果变化。

附加您的代码的原因是因为在用于清除第二个下拉列表的 for 循环中，不需要更新表达式，因为列表本身的大小正在减小，因此在每次迭代中，列表的长度都会减少，因此您无法清除整个列表。移除函数也应该在 if 条件之外以避免冗余。`function changecat() { var selectHTML = "";

var A = ["Soup", "Juice", "Tea", "Others"];
var B = ["Soup", "Juice", "Water", "Others"];
var C = ["Soup", "Juice", "Coffee", "Tea", "Others"];
var select = document.getElementById('category').options.length;

    for (var i = 0; i < select; ) {
        document.getElementById('category').options.remove(i);
    }

if (document.getElementById("meal").value == "A") {


    for (var i = 0; i < A.length; i++) {
        var newSelect = document.createElement('option');
        selectHTML = "<option value='" + A[i] + "'>" + A[i] + "</option>";
        newSelect.innerHTML = selectHTML;
        document.getElementById('category').add(newSelect);
    }
}

else if (document.getElementById("meal").value == "B") {

    for (var i = 0; i < B.length; i++) {
        var newSelect = document.createElement('option');
        selectHTML = "<option value='" + B[i] + "'>" + B[i] + "</option>";
        newSelect.innerHTML = selectHTML;
        document.getElementById('category').add(newSelect);
    }
}

else if (document.getElementById("project").value == "C") {

    for (var i = 0; i < C.length; i++) { 
        var newSelect = document.createElement('option');
        selectHTML = "<option value='" + C[i] + "'>" + C[i] + "</option>";
        newSelect.innerHTML = selectHTML;
        document.getElementById('category').add(newSelect);
    }
}

}