Using C# code inside JavaScript code

I always prefer this approach below ASPX page

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

In aspx.cs page Page_Load

Page.ClientScript.RegisterClientScriptBlock(Page.GetType(), "pageVars", string.Format("var TextBox1 = '{0}';", TextBox1.ClientID),true);

When required use the variable in JS file

function showValue() {
        alert(document.getElementById(TextBox1).value);
    }