Is there a way to capture errors while loading required resources on Tampermonkey?

Question

I have a tampermonkey script with a @require annotation for a library that provides me with certain tools for the script to work in a specific manner.

However, there are times in which this library is not available (it falls behind a login, so if no session is found, the @require returns a plain html with "unauthorized" explanation).

My objective would be to be able to capture if the resource is successfully loaded, and if it isn't provide with a handled error, which I'm not unable to do - I'm just getting the browser complaint about receiving a < symbol unexpectedly (because of the plain html resulting of the @require).

Do you have any ideas of how to tackle this problem? I've checked Tampermonkey documentation, but to no avail.

Thanks in advance

Answer 1

You could use GM.xmlHttpRequest to load the source from the url. Then check responseText of the result for the "unauthorized" text and then eval() or Function() to execute the code, see MDN for the difference between these two. You can do this every time the script is executed or you can cache the responseText with GM.getValue/GM.setValue

GM.xmlHttpRequest({
  method: "GET",
  url: "https://www.example.com/yourlibrary.js",
  onload: function (response) {
    if (!response.responseText.startsWith('unauthorized string')) {
      window.eval(response.responseText)
      main()
    } else {
      // ask for login
    }
  },
  onerror: function () {
    // ask for login
  }
})

function main () {
  // ... you userscript ...
}