Validate aspx texbox through Javascript - undefined or null reference

I am trying to validate a aspx textbpx with Javascript and am getting following error when if statement is executed in Javascript:

0x800a138f - JavaScript runtime error: Unable to get property 'value' of undefined or null reference

The following is the function in JavaScript in aspx page:

<script type="text/javascript">
 function myFunction(var1)
 if (document.getElementById("<%=TextBox1.ClientID%>").value == "") 
     alert("Please enter something");
 alert("Text box has value");

Following is aspx:

<div id="Div1">
    <asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="myFunction('TextBox1')" />
<asp:TextBox ID="TextBox1" runat="server" ClientIDMode="Static">TextBox Data</asp:TextBox>

I have tried, "ValueOf", "InnerText" also. I have also tried passing TextBox1 as avriable and still doesn't work.

Does anyone know why I am unable to access TextBox1 from JavaScript function???

Thank you


Try this

 if(document.getElementById('<%= yourtextboxid.ClientID%>').value != "")
       alert("has value!")
     } else {
      alert("please enter some thing");
      document.getElementById('<%= yourtextboxid.ClientID%>').focus();

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