jQuery selector add a string ,but selected get undefined value

Question

As the code below, they actually have 5 school drop-down and 5 departments drop-down list. So, it'll have five id, which is school_1 , school_2 , school_3 ...so on. The options are all same in these five school drop-down, same value, same text.

The department drop-down( id="dept_x" ) would show different departments list which belongs to the school that I selected in school drop-down list.

The school_1 connect with dept_1 ,and school_2 connect with dept_2 and ...so on.

Now, I have a problem with getting value in selected. The code var school_code = $("#school_" + num).val(); in the jQuery. I always get value undefined .

if I use selector "#school_1 or "#school_2 or other else, instead of "#school_" + num ,then it would work perfectly. It all do the same thing, so I choose to use for loop.

How do I fix this? or other better way?

Thank you so much!

The example HTML:

<select id="school_1" name="school_1" class="form-control">
    <option value="1001">school-A</option>
    <option value="1002">school-B</option>
    <option value="1003">school-C</option>
    <option value="1004">school-D</option>
</select>

<select id="dept_1" name="dept_1" class="form-control"></select>

The jQuery:

var school_num = '5';   //the value 5 is comes from a php variable and it's a string.
for(var num = 1; num <= parseInt(school_num); num++){

    num = String(num);
    //console.log("#school_" + num);

    $("#school_" + num).change(function(){
    $("#dept_" + num + " > option").remove();

        var school_code = $("#school_" + num).val();
        console.log(school_code);

        //then doing ajax to get department data in the database.

    });
}

Answer 1

I would suggest you to use a common class ie school to bind the change event handler. As store the arbitrary data in data-* custom attribute which can be fetched using .data(key) . Additionally .empty() can be used to remove all child nodes.

HTML

<select name="school_1" class="form-control school" data-id="1">
    <option value="1001">school-A</option>
    <option value="1002">school-B</option>
    <option value="1003">school-C</option>
    <option value="1004">school-D</option>
</select>

Script

$(".school").change(function(){
    var num = $(this).data('id');
    var dept = $("#dept_" + num);

    //remove options
    dept.empty();

    var school_code = $(this).val();
    console.log(school_code);

    //then doing ajax to get department data in the database.
});

Answer 2

Scope.

The value of your variable 'num' is read inside the .change() call - at that point, it will always be 5. Consider the pseudo-code version:

• loop through each element, 1-5
• assign change handler
• end loop
• later: change handler hits - num is 5

Fix this by using this inside the handler:

var school_code = $(this).val();

You'll have to do something similar for dept.

Answer 3

You need to use a closure to ensure the num value within the change event handler is what you expect it to be, and not the last value in the array - as the for loop will have finished long before any change event has fired.

var school_num = '5';
for(var num = 1; num <= parseInt(school_num, 10); num++){
    (function(num) {
        $("#school_" + num).change(function(){
            $("#dept_" + num + " > option").remove();
            var school_code = $("#school_" + num).val();

            //then doing ajax to get department data in the database.
       });  
    })(num)
}

That being said you can massively improve the logic by using common classes and a single event handler. You can then use DOM traversal from the first select to find the related select . This will completely remove the need for the loop. Try this:

 $('.school').change(function() { var $dept = $(this).next('.dept').empty() var school_code = $(this).val(); console.log(school_code) //then doing ajax to get department data in the database. $dept.append('<option>Foo</option>'); // just an example }); 

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <select class="school" name="school_1" class="form-control"> <option value="1001">school-A</option> <option value="1002">school-B</option> <option value="1003">school-C</option> <option value="1004">school-D</option> </select> <select class="dept" name="dept_1" class="form-control"></select> <select class="school" name="school_2" class="form-control"> <option value="1001">school-A</option> <option value="1002">school-B</option> <option value="1003">school-C</option> <option value="1004">school-D</option> </select> <select class="dept" name="dept_2" class="form-control"></select>