To preselect a <select> list item using JavaScript, use the selectedIndex property. Add an index of what you want to be selected for this property.
Here, under the <script> tag, add_select_id is the id of the <select> tag, whereas add_item_index is the index in numbers. This index is the list item index, you need to add for the item you want to be preselected.
Example
You can try to run the following code to preselect a list item using JavaScript −
<!DOCTYPE html>
<html>
<head>
<title>Preselect List Item</title>
</head>
<body>
<p> Select any one:</p>
<form>
<select id="dropdown" name="dropdown" class="form-control">
<option value = "Java">Java</option>
<option value = "Discrete Mathematics">Discrete Mathematics</option>
<option value = "Digital Electronics">Digital Electronics</option>
</select>
</form>
<script>
document.getElementById("dropdown").selectedIndex = "2";
</script>
</body>
</html>