How to use viewstate value in Javascript in asp.net?

Use RegisterHiddenField than mixing server/js codes:

JavaScript:

<script language="javascript" type="text/javascript">
    var vCode = document.getElementById("vCode");
alert(vCode);
</script>

Code behind:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    ViewState("code") = "EE"
    Page.ClientScript.RegisterHiddenField("vCode", ViewState("code"))
End Sub

For more have look on following threads:
Some CP Articles:
ASP.NET Internals: Viewstate and Page Life Cycle[^]
Beginner's Guide To View State [^]
Access ViewState Across Pages[^]

Other site:
Maintaining the ViewState[^]
ViewState in ASP.NET[^]