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Program Lifecycle

Deno supports browser compatible lifecycle events:

  • load: fired when the whole page has loaded, including all dependent resources such as stylesheets and images.
  • beforeunload: fired when the event loop has no more work to do and is about to exit. Scheduling more asynchronous work (like timers or network requests) will cause the program to continue.
  • unload: fired when the document or a child resource is being unloaded.
  • unhandledrejection: fired when a promise that has no rejection handler is rejected, ie. a promise that has no .catch() handler or a second argument to .then().

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, this event cannot be canceled. Listeners for beforeunload need to be synchronous and can be cancelled to keep the program running. Listeners for unload events need to be synchronous and 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("beforeunload", handler);

globalThis.addEventListener("unload", handler);

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

globalThis.onbeforeunload = (e: Event): void => {
  console.log(`got ${e.type} event in onbeforeunload 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("beforeunload", handler);
globalThis.addEventListener("unload", handler);

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

globalThis.onbeforeunload = (e: Event): void => {
  console.log(`got ${e.type} event in onbeforeunload 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 onbeforeunload event in event handler (imported)
got onbeforeunload event in event handler (main)
got onbeforeunload event in onbeforeunload 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, onbeforeunload 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 defined onload, onbeforeunload, onunload event handlers will be executed. It is preferable to use addEventListener when possible for this reason.

beforeunload Example

// beforeunload.js
let count = 0;

console.log(count);

globalThis.addEventListener("beforeunload", (e) => {
  console.log("About to exit...");
  if (count < 4) {
    e.preventDefault();
    console.log("Scheduling more work...");
    setTimeout(() => {
      console.log(count);
    }, 100);
  }

  count++;
});

globalThis.addEventListener("unload", (e) => {
  console.log("Exiting");
});

count++;
console.log(count);

setTimeout(() => {
  count++;
  console.log(count);
}, 100);

Running this program will print:

$ deno run beforeunload.js
0
1
2
About to exit...
Scheduling more work...
3
About to exit...
Scheduling more work...
4
About to exit...
Exiting

This has allowed us to polyfill process.on("beforeExit") in the Node compatibility layer.

unhandledrejection event Example:

This release adds support for the unhandledrejection event. This event is fired when a promise that has no rejection handler is rejected, ie. a promise that has no .catch() handler or a second argument to .then().

// unhandledrejection.js
globalThis.addEventListener("unhandledrejection", (e) => {
  console.log("unhandled rejection at:", e.promise, "reason:", e.reason);
  e.preventDefault();
});

function Foo() {
  this.bar = Promise.reject(new Error("bar not available"));
}

new Foo();
Promise.reject();

Running this program will print:

$ deno run unhandledrejection.js
unhandled rejection at: Promise {
  <rejected> Error: bar not available
    at new Foo (file:///dev/unhandled_rejection.js:7:29)
    at file:///dev/unhandled_rejection.js:10:1
} reason: Error: bar not available
    at new Foo (file:///dev/unhandled_rejection.js:7:29)
    at file:///dev/unhandled_rejection.js:10:1
unhandled rejection at: Promise { <rejected> undefined } reason: undefined

This API will allow us to polyfill process.on("unhandledRejection") in the Node compatibility layer in future releases.