从另一个下拉值的选择中动态填充下拉列表

Question

我的要求是，对于“膳食”下拉列表中的选择，第二个下拉列表“类别”应该动态填充与第一个下拉列表中的选择相关的值。然后根据在餐点下拉菜单中选择的内容，列表应更改类别。我编写了以下 Javascript 函数，但我得到的输出不是新填充第二个下拉列表。在更改选择时，新列表只是被附加到旧列表中。

function changecat() {
    var selectHTML = "";

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

    if (document.getElementById("meal").value == "A") {
        var select = document.getElementById('category').options.length;

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

        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") {
        var select = document.getElementById('category').options.length;

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

        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") {
        var select = document.getElementById('category').options.length;

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

        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);
        }
    }
}

---

HTML-  
<select name="meal" id="meal" onchange="changecat();">
    <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">
    <option value="">Select</option>
</select>

Answer 1

也许对你有帮助

JSFiddle : DEMO

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;
        }
    }

其实 JavaScript 支持一种循环，即 for...in 循环。

for...in 循环仅迭代可枚举的属性。对象 从 Array 和 Object 等内置构造函数创建 从 Object.prototype 继承的不可枚举属性和 String.prototype，例如 String 的 indexOf() 方法或 Object 的 toString() 方法。循环将遍历所有可枚举 对象本身的属性以及对象从其继承的属性 构造函数的原型（更接近对象的属性） 原型链覆盖原型的属性）。

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。

了解更多Link

Answer 2

您可以使用onchange 事件并使用带有从第一个下拉列表中选择的值的 switch 语句，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" 和第二个下拉菜单来查看每个选择的实时结果变化。

Answer 3

附加代码的原因是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小，因此您无法清除整个列表。此外，删除函数应该在 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);
    }
}

}