Retrieve Session value in Javascript from Code behind file Aspx.cs

Retrieve Session value in Javascript from Code behind file Aspx.cs

I am storing some value in session through code behind file named
Index4.aspx.cs by using this code: Session[txtUsername.Text.ToUpper() +
"ChannelID_BC"] = 1111110;
But now how to retrieve it in Index$.aspx page through JavaScript