How to make the select tag expand by default in javascript

In HTML, the element is used to create drop-down lists, usually used in web forms. However, many times we need to automatically expand this drop-down list when the page loads. While this can be accomplished in HTML by setting the "selected" attribute, sometimes we need more advanced settings in JavaScript to suit our needs. This article will introduce how to use JavaScript to expand the element.

Implementation method:

1. Use the autofocus attribute of HTML5

The easiest way is to use the autofocus attribute of HTML5. This attribute can automatically set focus on the specified input box, button, drop-down list and other elements after the page is loaded. To make a automatically expand, you can set it as an autofocus element. The code is as follows:

<select autofocus>
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>

2. Using JavaScript code

When we need to expand the

element when the page loads, the autofocus attribute may not be able to achieve our needs. At this time, we can use JavaScript code to achieve this. The code is as follows:

<select id="mySelect">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>
<script>
  document.getElementById('mySelect').size = document.getElementById('mySelect').length; 
</script>

The above code automatically expands the drop-down list by setting its size (size) to its number of options (length). Of course, you can also use other methods to achieve the same effect, such as setting the options attribute, or using CSS styles, etc.

3. Expand under specific conditions

Sometimes we need to expand

under specified conditions, such as when an event is triggered, or when a certain value When conditions are met. At this time we need to use JavaScript code to operate. The code is as follows:

<select id="mySelect">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>
<script>
  document.getElementById('mySelect').addEventListener('click', function() {
    this.size = this.length;
  });
</script>

The above code listens to the click event of through the addEventListener() method, and sets its size to the number of options after clicking, thereby expanding the drop-down list.

Note:

Although using JavaScript code can easily realize the expansion of

elements, we need to pay attention to some details:

1. Using JavaScript code The
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