未捕获的TypeError：无法读取未定义的属性'toLowerCase'

Question

I'm getting this error and it is originating from jquery framework. When i try to load a select list on document ready i get this error. I can't seem to find why i'm getting this error.

It works for the change event, but i'm getting the error when trying to execute the function manually.

Uncaught TypeError: Cannot read property 'toLowerCase' of undefined -> jquery-2.1.1.js:7300

Here is the code

$(document).ready(function() {
    $("#CourseSelect").change(loadTeachers);
    loadTeachers();
});

function loadTeachers() {
    $.ajax({ 
        type: 'GET', 
        url: '/Manage/getTeachers/' + $(this).val(), 
        dataType: 'json', 
        cache: false,
        success:function(data) { 
            $('#TeacherSelect').get(0).options.length = 0;    
            $.each(data, function(i, teacher) {
                var option = $('<option />');
                option.val(teacher.employeeId);
                option.text(teacher.name);
                $('#TeacherSelect').append(option);
            });
        },         
        error: function() { 
            alert("Error while getting results"); 
        }
    });
}

Answer 1

When you call loadTeachers() on DOMReady the context of this will not be the #CourseSelect element.

You can fix this by triggering a change() event on the #CourseSelect element on load of the DOM:

$("#CourseSelect").change(loadTeachers).change(); // or .trigger('change');

Alternatively can use $.proxy to change the context the function runs under:

$("#CourseSelect").change(loadTeachers);
$.proxy(loadTeachers, $('#CourseSelect'))();

Or the vanilla JS equivalent of the above, bind() :

$("#CourseSelect").change(loadTeachers);
loadTeachers.bind($('#CourseSelect'));

Answer 2

I had the same problem, I was trying to listen the change on some select and actually the problem was I was using the event instead of the event.target which is the select object.

INCORRECT :

$(document).on('change', $("select"), function(el) {
    console.log($(el).val());
});

CORRECT :

$(document).on('change', $("select"), function(el) {
    console.log($(el.target).val());
});

Answer 3

It fails " when trying to execute the function manually " because you have a different 'this'. This will refer not to the thing you have in mind when invoking the method manually, but something else, probably the window object, or whatever context object you have when invoking manually.

Answer 4

It causes the error when you access $(this).val() when it called by change event this points to the invoker ie CourseSelect so it is working and and will get the value of CourseSelect . but when you manually call it this points to document. so either you will have to pass the CourseSelect object or access directly like $("#CourseSelect").val() instead of $(this).val() .

Answer 5

This Works For me !!!

Call a Function without Parameter

$("#CourseSelect").change(function(e1) {
   loadTeachers();
});

Call a Function with Parameter

$("#CourseSelect").change(function(e1) {
    loadTeachers($(e1.target).val());
});

Answer 6

your $(this).val() has no scope in your ajax call, because its not in change event function scope

May be you implemented that ajax call in your change event itself first, in that case it works fine. but when u created a function and calling that funciton in change event, scope for $(this).val() is not valid.

simply get the value using id selector instead of

$(#CourseSelect).val()

whole code should be like this:

 $(document).ready(function () 
{
    $("#CourseSelect").change(loadTeachers);
    loadTeachers();
});

function loadTeachers()
{
    $.ajax({ type:'GET', url:'/Manage/getTeachers/' + $(#CourseSelect).val(), dataType:'json', cache:false,
        success:function(data)
        { 
            $('#TeacherSelect').get(0).options.length = 0;    

            $.each(data, function(i, teacher) 
            {
                var option = $('<option />');
                option.val(teacher.employeeId);
                option.text(teacher.name);
                $('#TeacherSelect').append(option);
            });
        }, error:function(){ alert("Error while getting results"); }
    });
}