Unknown(Script Error)

This happens when your JS code is hosted on a different domain (i.e. a CDN). To fix, set Access-Control-Allow-Origin: * on the JS file, and crossorigin="anonymous" on the <SCRIPT> tag.

If the script errors are coming from Javascript that you don't maintain, you can suppress them using the ignoreMessages option in Rollbar.js

It is normal to still get some of these errors even after following the steps below and setting up CORS, because almost everyone has third party JS on their site that isn't set up with the CORS headers.

Full explanation

"Script Error" happens when an uncaught JavaScript error crosses domain boundaries in violation of the cross-origin policy. For example, if you host your JavaScript code on a CDN, any uncaught errors (errors that bubble up to the window.onerror handler, instead of being caught in try-catch) will get reported as simply "Script error" instead of containing useful information. This is a browser security measure intended to prevent passing data across domains that otherwise wouldn't be allowed to communicate. It's implemented in Firefox and Chrome.

To get the real error messages, do the following:

1. Send the Access-Control-Allow-Origin header

Setting the Access-Control-Allow-Origin header to * signifies that the resource can be accessed properly from any domain. You can replace * with your domain if necessary, for example Access-Control-Allow-Origin: www.example.com. However, handling multiple domains gets tricky, and may not be worth the effort if using a CDN due to caching issues that may arise. See more here.

Here are some examples on how to set this header in various environments:

Apache

In the folders where your JavaScript files will be served from, create an .htaccess file with the following contents:

Header add Access-Control-Allow-Origin "*"

Nginx

Add the add_header directive to the location block that serves your JavaScript files:

location ~ ^/assets/ {
    add_header Access-Control-Allow-Origin *;
    ...
}

HAProxy

Add the following to your asset backend where JavaScript files are served from:

rspadd Access-Control-Allow-Origin: *

ExpressJS

Add the following to your app:

app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
});

More examples for different platforms here

IIS

In IIS, you can use the following code in your web.config:

<location path="assets">
    <system.webServer>
        <httpProtocol>
            <customHeaders>
                <add name="Access-Control-Allow-Origin" value="*" />
            </customHeaders>
        </httpProtocol>
    </system.webServer>
</location>

2. Set crossorigin="anonymous" on the script tag

In your HTML source, for each of the scripts that you've set the access-control-allow-origin header for, set crossorigin="anonymous" on the <SCRIPT> tag.

Make sure you verify that the header is being sent for the script file before adding the crossorigin property on the script tag. In Firefox, if the crossorigin attribute is present but the Access-Control-Allow-Origin header is not, the script won't be executed.