How does JQuery realize the dynamic change function of the value of the drop-down box?-Front-end Q&A-php.cn

How does JQuery realize the dynamic change function of the value of the drop-down box?

PHPz

Release： 2023-04-17 13:54:23

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JQuery is a popular JavaScript library that is widely used in web development. Among them, the drop-down box is a widely used interactive element. In some scenarios, the web page needs to dynamically change the value of the drop-down box based on user operations. In this article, we will introduce how to use JQuery to dynamically change the value of a drop-down box.

First, add a reference to the JQuery library at the head of the web page. You can use CDN or local introduction. As shown below:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JQuery下拉框的值动态改变</title>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
</head>
<body>
    <select id="selectBox">
        <option value="value1">选项一</option>
        <option value="value2">选项二</option>
        <option value="value3">选项三</option>
    </select>
    <button id="changeBtn">点击改变选项</button>
</body>
</html>

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In the above code example, we created a drop-down box and a button. The ID of the drop-down box is selectBox, which contains three options; the ID of the button is changeBtn, which is used to trigger the dynamic change of the value of the drop-down box.

Next, add the JQuery code. First, we need to capture the click event of the button. When the user clicks the button, we will use JQuery to dynamically change the value of the drop-down box. The code is as follows:

$(function () {
    $("#changeBtn").click(function () {
        // TODO: 下拉框动态改变值的实现
    });
});

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Among them, $(function () {}) is the abbreviation of window.onload. After the DOM element is loaded, we will capture the button click event and execute the code that dynamically changes the value of the drop-down box. Next, we will implement the code to dynamically change the value of the drop-down box at the location of the TODO comment.

We can modify the options in the drop-down box inside the click event handler function. JQuery provides the val() function for getting or setting the value of a form element. The drop-down box is a form element, and we can use this function library to dynamically modify the value of the drop-down box. The code is as follows:

$(function () {
    $("#changeBtn").click(function () {
        // 获取下拉框中的选项值
        var selectValue = $("#selectBox").val();

        // 根据选项值修改下拉框中的选项
        if (selectValue === "value1") {
            $("#selectBox").val("value2");
        } else if (selectValue === "value2") {
            $("#selectBox").val("value3");
        } else if (selectValue === "value3") {
            $("#selectBox").val("value1");
        }
    });
});

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In the above code example, we first obtain the currently selected value of the drop-down box and store it in the selectValue variable. Next, we decide to modify the options in the drop-down box based on the option value. If "value1" is currently selected, we will change the value selected in the drop-down box to "value2"; if "value2" is currently selected, we will change the value selected in the drop-down box to "value3"; If "value3" is selected, we will change the selected value in the drop-down box to "value1".

Finally, after updating the options of the drop-down box, we need to use JQuery's change() function to manually trigger the change event of the drop-down box. This will update the value of the dropdown box on the web page and trigger the associated event handler. The code is as follows:

$(function () {
    $("#changeBtn").click(function () {
        // 获取下拉框中的选项值
        var selectValue = $("#selectBox").val();

        // 根据选项值修改下拉框中的选项
        if (selectValue === "value1") {
            $("#selectBox").val("value2");
        } else if (selectValue === "value2") {
            $("#selectBox").val("value3");
        } else if (selectValue === "value3") {
            $("#selectBox").val("value1");
        }

        // 手动触发下拉框的改变事件
        $("#selectBox").change();
    });
});

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In the above code example, we manually triggered the change event of the drop-down box after modifying the option value of the drop-down box. This will ensure that our updated options display and take effect correctly on the page.

To sum up, we use JQuery to dynamically change the value of the drop-down box. This allows us to more flexibly control user operations in web development and improves the web interaction experience.

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