Why would there be a need to call a JavaScript function from code behind?

Question

I just been looking around online about calling a JavaScript function from code behind, from examples that I have seen you RegisterStartupScript method on a click event. But why would you want or need to do that instead of just wiring it up the OnClientClick event? Is there ever a need to call a JavaScript function from code behind?

Answer 1

RegisterStartupScript is one of many options for an infinite number of scenarios. In the end, anything you can do with RegisterStartupScript can be done another way. I used to consider it a convenience, now I avoid it, separation of concerns and such higher stages of "enlightenment".

Mainly where I see RegisterStartupScript still in use is with custom controls that are expected to wire themselves up without the end-user knowing anything about them. See AjaxControlToolKit, UpdatePanel, ScriptManager, etc. They all require javascript but for obvious reasons do not expect you to include their client-side scripts or register them.

Random Scenario:

if (User.Identity.Name == "Frank")
    RegisterStartupScript(this, GetType(), "Frank", "alert("Hey Frank, you owe me money!");

Alternative Scenario, have the server-side set a hidden field.

<input type="hidden" id="name" value="<%= User.Identity.Name %>" />

<script type="text/javascript">
    $(document).ready(function() {
        if ($("#name").val() == "Frank")
            alert("Hey Frank, you owe me money!");
    });
</script>