Listeners for load events can be asynchronous and will be awaited. Listeners for unload events need to be synchronous. Both events cannot be cancelled.

main.ts

A couple notes on this example:

In other words, you can use addEventListener to register multiple "load" or "unload" event handlers, but only the last loaded onload or onunload event handlers will be executed. It is preferable to use addEventListener when possible for this reason.