Show JavaScript alert() or execute other code from Code-Behind

I find that sometimes I need to show the user a JavaScript alert for something I don’t want them to be able to miss and I only want to show it based on some server-side (Code-Behind) code. This will work for most any chunk of JavaScript code as well.

Below is the code that you can put in your Page_Load()

if (!Page.ClientScript.IsClientScriptBlockRegistered("MyMadeUpNameHere"))
{
    string jscript = "alert('test here');";
    Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "MyMadeUpNameHere", jscript, true);
}

Notice that MyMadeUpNameHere is some unique string that allows you to not have multiple copies of your code on the page. This ensures that you will only have it execute once. If you don’t want that functionality then don’t do the check.

In this example, the user will see the ‘test here’ in a JavaScript alert (Message Box).

Note, that the RegisterClientScriptBlock puts the JavaScript immediately below the opening form tag. If your code references any form elements or any other elements on the page this is most likely going to be too early since the page has not finished loading. This makes this method good for simple things like I did here that doesn’t really care about the page, or you can use it to register JavaScript functions that is executed later in the page or in an event handler.

If you want your code to execute LATER then I recommend using the RegisterStartupScript and IsStartUpScriptRegistered methods instead. This code executes at the end of the page after all the items on the page have finished loading. Here is the same code as above, but with the StartUp versions.

if (!Page.ClientScript.IsStartupScriptRegistered("MyMadeUpNameHere"))
{
    string jscript = "alert('test here');";
    Page.ClientScript.RegisterStartupScript(this.GetType(), "MyMadeUpNameHere", jscript, true);
}

Like I said before, if you had more complex logic you could put it earlier in the page and just execute the function using the RegisterStartupScript. You can also put your complex logic in functions in JScript include and use the RegisterClientScriptInclude. See here for more details on that.

I don’t know why I can never remember which methods to use for showing a JavaScript alert or executing other JavaScript code when the page loads, so I am writing in down for me and everyone else to have.