Javascript validation return false still page was postback?

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i have javascript validation for file upload control on button OnClintclick event.
javascript function return false still button click event firing...how to avoid postback when javascript returns false?

thankyou

What I have tried:

My js code is

<script language="javascript" type="text/javascript">
        function checkRequiredFields() {


            var imgpath = document.getElementById('FileUpload1').value;
            if (imgpath == "") {
                alert("please select a file");
                document.file.FileUpload1.focus();
                return false;
            }
            else {
                alert("success");
            }

            var array = ['zip', 'jpg', 'jpeg', 'doc', 'docx', 'xls', 'xlsx', 'pdf', 'txt'];
            var xyz = document.getElementById("FileUpload1");
            var Extension = xyz.value.substring(xyz.value.lastIndexOf('.') + 1).toLowerCase();

            if (array.indexOf(Extension) <= -1) {
                alert("Selected file does not meet the upload criteria.");
                document.form.FileUpload1.focus();
                return false;
            }
             else {
                 alert("success");
            }

           }
       </script>

button control is

<asp:Button ID="CaseAttachments_btnUpload" runat="server" CssClass="FA_TextButton" Text="Upload" OnClientClick="return checkRequiredFields();" OnClick="MyButton_Click" />

Posted 19-Apr-16 1:49am

User-11630313

Updated 19-Apr-16 2:49am

Passion4Code

v2

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Comments

Karthik_Mahalingam 19-Apr-16 8:10am

Place debugger and check in which line you are getting error

ZurdoDev 19-Apr-16 8:15am

Make certain it is returning false. Also, it seems like I've sometimes had to set UseSubmitBehavior to false for it to actually work. Not sure why. Perhaps try that as well.

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Karthik_Mahalingam · Answer 1 · 2016-04-19T02:49:00

This Error will happen whenever there is an error in the javascript code ( syntax error or run time error )
If there is an error (Syntax or run time), and if unhanded it will return as true.
Try to do check the below :
Place a debugger in the first line of your javascript function

JavaScript

function checkRequiredFields() {
           debugger; // debugger to stop code execution on this particular line
           var imgpath = document.getElementById('FileUpload1').value;

To debug you can either use Chrome browser or IE by Enabling the Developer Tools, which can be done by

Chrome- Open the developer tools by pressing F12 or Menu->Tools (More tools) -> Developer tools
IE

F12

Try to hit the function by some event ( say button click ). now you can debug line by line to Find where the exception has been raised.
The error will be displayed in the Console Tab of the Developer Tools window.