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。然后根据在膳食下拉列表中选择的内容，列表应该改变类别。我已经编写了以下 Javascript 函数，但我得到的输出不是新鲜填充第二个下拉列表。在更改选择时，新列表只是附加到旧列表。

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>

最佳答案

希望这对你有帮助。

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;
        }
    }

JavaScript 中有一个循环(for...in 循环)，在这种情况下可以帮助您

A for...in loop only iterates over enumerable properties. Objects created from built–in constructors like Array and Object have inherited non–enumerable properties from Object.prototype and String.prototype, such as String's indexOf() method or Object's toString() method. The loop will iterate over all enumerable properties of the object itself and those the object inherits from its constructor's prototype (properties closer to the object in the prototype chain override prototypes' properties).

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Link

关于javascript - 从选择另一个下拉值动态填充下拉列表，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30232146/