使用jQuery判断文件输入字段是否为空

I have a PHP form with a file input field, and two text input fields. What I want to do is use jQuery to check if the file input field is empty, and if it's not, then show a specific div, but the following code does not work:

<input type="file" name="blog-entry-image" id="blog-entry-image" />

var fileInput = $.trim($("#blog-entry-image").val());
if (fileInput.length > 0) {
        $("#new-blogentry").click(function() {
        $("#sending").show();
        return true;
    }); 
}

"new-blogentry" is the submit button, and "sending" is a div with an animated gif.

<input type="file" name="blog-entry-image" id="blog-entry-image" />

$(document).ready(function (){
    $("#blog-entry").click(function() {
        var fileInput = $.trim($("#blog-entry-image").val());
        if (fileInput && fileInput !== '') {      
                $("#sending").show();
                return true;    
        }
  });
});

Change fileInput.length > 0 to

fileInput != "" && fileInput.trim() != ""

You have things backwards. You need to get the file input value after the user clicks.

$('#new-blogentry').click(function() {
    var fileInput = $.trim($("#blog-entry-image").val());
    if (fileinput.length > 0) {
        ...
    }
});

if($('#new-blogentry').val()==''))

is enough to check if the file input is empty

I suggest you to use jquery validator plugin for all frontend based validation http://jqueryvalidation.org/