Program lifecycle

Deno supports browser compatible lifecycle events: load and unload. You can use these events to provide setup and cleanup code in your program.

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

Example:

main.ts

import "./imported.ts";

const handler = (e: Event): void => {
  console.log(`got ${e.type} event in event handler (main)`);
};

globalThis.addEventListener("load", handler);

globalThis.addEventListener("unload", handler);

globalThis.onload = (e: Event): void => {
  console.log(`got ${e.type} event in onload function (main)`);
};

globalThis.onunload = (e: Event): void => {
  console.log(`got ${e.type} event in onunload function (main)`);
};

console.log("log from main script");

imported.ts

const handler = (e: Event): void => {
  console.log(`got ${e.type} event in event handler (imported)`);
};

globalThis.addEventListener("load", handler);
globalThis.addEventListener("unload", handler);

globalThis.onload = (e: Event): void => {
  console.log(`got ${e.type} event in onload function (imported)`);
};

globalThis.onunload = (e: Event): void => {
  console.log(`got ${e.type} event in onunload function (imported)`);
};

console.log("log from imported script");

A couple notes on this example:

addEventListener and onload/onunload are prefixed with globalThis, but you could also use self or no prefix at all. It is not recommended to use window as a prefix.
You can use addEventListener and/or onload/onunload to define handlers for events. There is a major difference between them, let's run the example:

$ deno run main.ts
log from imported script
log from main script
got load event in event handler (imported)
got load event in event handler (main)
got load event in onload function (main)
got unload event in event handler (imported)
got unload event in event handler (main)
got unload event in onunload function (main)

All listeners added using addEventListener were run, but onload and onunload defined in main.ts overrode handlers defined in imported.ts.

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.