How to set default value for bootstrap drop-down list

Get the value from the drop-down box and set the default value:
##

<select id="test">
<option value="1">选项一<option>
<option value="2">选项一<option>
...
<option value="n">选项N<option>
</select>

The so-called jQuery operation "select", To be more precise, jQuery should control "option". Look at the jQuery code below:

//Get the value of the first option

$('#test option:first').val();

//The value of the last option

$('#test option:last').val();

//Get the value of the second option

$('#test option:eq(1)').val();

//Get the selected value

$('#test').val();
$('#test option:selected').val();

//Set the option with a value of 2 to the selected state

$(‘#test’).attr(‘value’,’2’);

//Set the last option to be selected

$('#test option:last').attr('selected','selected');
$("#test").attr('value' , $('#test option:last').val());
$("#test").attr('value' , $('#test option').eq($('#test option').length - 1).val());

//Get the length of the select

$('#test option').length;

//Add an option

$("#test").append("<option value=&#39;n+1&#39;>第N+1项</option>");
$("<option value=&#39;n+1&#39;>第N+1项</option>").appendTo("#test");

//Add and remove Selected item

$('#test option:selected').remove();

//Delete the selected item (the first item is deleted here)

$('#test option:first').remove();、

//The specified value is deleted

$('#test option').each(function(){
if( $(this).val() == '5'){
$(this).remove();
}
});
$('#test option[value=5]').remove();

//Get the label of the first Group

$('#test optgroup:eq(0)').attr('label');

//Get the value of the first option under the second group

$('#test optgroup:eq(1) : option:eq(0)').val();

//select is selected by default

$("#arrangeClass").find("option[value="+reim.classId+"]").prop("selected",true);

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