Set Focous on a Textfield in asp.net using javascript

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ASP.NET

Javascript

I am trying to put some validation on my registration form in asp.net using JavaScript. but it didn't work the way i want. i want that if user leave a text-field empty an alert message pops up and then set force user to fill that field by using focus.

I write this code, but the problem is that after showing alert message, it didn't set focus on TextBox2.

<asp:TextBox ID="TextBox2" runat="server" onblur="ValidateTextNotNull()"></asp:TextBox>

function ValidateTextNotNull()
     {
         var Firstname = document.getElementById('TextBox2').value;
         var textLength = Firstname.length;
         if (textLength == 0) {
             alert("show your message");
             document.getElementById('TextBox2').focus();
         }

     }