How to use jQuery to implement an echo drop-down box

Drop-down boxes are often used in web pages to improve the user experience. In some cases, we need to set the drop-down box to echo state, that is, when the page is reloaded, the drop-down box should display the previously selected value. This article will introduce how to use jQuery to implement an echo drop-down box.

1. HTML Template

First, we need to prepare a simple HTML template that contains a drop-down box. Set the value and text for each option in the drop-down box.

<select id="mySelect">
  <option value="1">选项一</option>
  <option value="2">选项二</option>
  <option value="3">选项三</option>
  <option value="4">选项四</option>
</select>

2. Set Cookie

We will use Cookie to store the selected value. A cookie is a small file used to store client data. We can read and write cookies using JavaScript's document.cookie object.

The following is a JavaScript function that sets Cookie:

function setCookie(key, value, expires) {
  var date = new Date();
  date.setTime(date.getTime() + expires * 24 * 60 * 60 * 1000);
  var expires = "expires=" + date.toUTCString();
  document.cookie = key + "=" + value + "; " + expires;
}

This function has three parameters, namely key, value and expires. key is the name of the cookie, value is the value of the cookie, and expires is the expiration time of the cookie, in days. In this function, we convert the expiration time to UTC time and write the cookie to the client using the document.cookie object.

3. Get Cookie

We also need to write a function to get Cookie. The following is the JavaScript function to obtain Cookie:

function getCookie(key) {
  var cookies = document.cookie.split("; ");
  for (var i = 0; i < cookies.length; i++) {
    var cookie = cookies[i].split("=");
    if (cookie[0] == key) {
      return cookie[1];
    }
  }
  return "";
}

This function receives a parameter key, which represents the name of the Cookie to be obtained. The function reads all cookies in document.cookie into an array, and then traverses the array to find the specified cookie. If the specified cookie is found, its value is returned.

4. Echo the drop-down box

Now we can use jQuery to implement the echo drop-down box. Paste the following code into the JavaScript of the page:

$(document).ready(function() {
  var selectedValue = getCookie("selectedValue");
  $("#mySelect").val(selectedValue);
});

$("#mySelect").change(function() {
  var selectedValue = $(this).val();
  setCookie("selectedValue", selectedValue, 365);
});

In this code, we use jQuery’s $(document).ready() method to check whether the page has finished loading. If so, we will read the last selected value stored in the cookie and set it as the selected value of the dropdown box.

We also use jQuery’s $("#mySelect").change() method to listen for changes in the drop-down box. When the value of the drop-down box changes, we will get the new selected value and write it to the cookie.

Now, the echo drop-down box is completed. When the page reloads, the dropdown should display the previously selected value.

Summary

This article introduces how to use jQuery to implement an echo drop-down box. We used cookies to store the selected value and jQuery to read and write cookies. I hope this article can help you implement echo drop-down boxes and improve user experience.

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