Javascript t、 所以我没有更新它；） $('.target').change(function() { alert('Handler for .change() called.'); }); $('#target').change(function() {

t、 所以我没有更新它；）

$('.target').change(function() {
  alert('Handler for .change() called.');
});

$('#target').change(function() {
  alert('Handler for .change() called.');
});

<form>
    <select id="target" class="target">
        <option name="pp" value="6">6</option>
        <option name="pp" value="12">12</option>
    </select>
 </form>

$(window).load(function() {
    $("img.gall_img").each(function() { // iterate through all img of class gall_img
        var imgWidth = $(this).width(); // "$(this)" to access jQuery width() func
        var divWidth = $(this).closest('div').width();
        //alert(imgWidth + " and " + divWidth); // for debugging
        if(imgWidth > divWidth) {
            var position = Math.round(-1 * ((imgWidth/2) - (divWidth/2))); // 
            position = position + "px"; // make position format "123px".
                $(this).css("left", position); // "$(this)" to access jQuery css() func
            }
    });
});


$("#target").change(function() {
  alert('Handler for .change() called.');
});

$(document).ready(function () {
  $('#target').change(function() {
    alert('Handler for .change() called.');
  });
});

$(window).load(function () {

//whatever code is already here

$('.target').change(function() {
  alert('Handler for .change() called.');
});

}); //end of $(window).load function