Sys.Application.add_load() vs. $(document).ready() vs. pageLoad()

If you have control over the code-behind, you can register the JavaScript to run at startup via something like:

this.Page.ClientScript.RegisterStartupScript(
    this.GetType(), 
    "StartupScript", 
    "Sys.Application.add_load(function() { functioncall(); });", 
    true);

As long as your component has been loaded via Sys.Application.add_init() you should be fine…