为什么我无法使用javascript从输入中获取价值？

Question

当我按下click here时，它将是警告空白和空白值。

我想知道为什么不提醒400和400

test1.php

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<form id="first_image_data_url_fid" style="display: block;">
    <input type="text" name="first_image_data_url" id="first_image_data_url"/>

    <input type="text" name="first_image_data_dimension_width" id="first_image_data_dimension_width"/>
    <input type="text" name="first_image_data_dimension_height" id="first_image_data_dimension_height"/>
</form>
<span id="mySpan_check_image_data_width_height"></span>

<div onclick="test_fn()">click here</div>

<script language="JavaScript" type="text/javascript">
function test_fn()
{
var first_image_data_url = "https://pbs.twimg.com/profile_images/839721704163155970/LI_TRk1z_400x400.jpg";
document.getElementById("first_image_data_url").value = first_image_data_url;
document.getElementById("first_image_data_dimension_width").value = "";
document.getElementById("first_image_data_dimension_height").value = "";
$.ajax
(
    {
        url: 'test2.php',
        type: 'POST',
        data: $('#first_image_data_url_fid').serialize(),
        cache: false,
        success: function (data) {
            $('#mySpan_check_image_data_width_height').html(data);
            //get_first_image_data_width_height();
        }
    }
)

var item = get_first_image_data_width_height();
}
</script>


<script>
function get_first_image_data_width_height(){
    var item;
    var first_image_data_dimension_width_val = document.getElementById("first_image_data_dimension_width").value;
    alert(first_image_data_dimension_width_val);

    var first_image_data_dimension_height_val = document.getElementById("first_image_data_dimension_height").value;
    alert(first_image_data_dimension_height_val);

var item = "0";

    return item;
}
</script>

test2.php

<?PHP
session_start();
include("connect.php");
$first_image_data_url = mysqli_real_escape_string($db_mysqli,$_POST['first_image_data_url']);

$first_image_data_dimension = getimagesize($first_image_data_url);
$first_image_data_dimension_width = $first_image_data_dimension[0];
$first_image_data_dimension_height = $first_image_data_dimension[1];
?>

<script>
document.getElementById("first_image_data_dimension_width").value = "<?PHP echo $first_image_data_dimension_width; ?>";
document.getElementById("first_image_data_dimension_height").value = "<?PHP echo $first_image_data_dimension_height; ?>";
</script>