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doc: add snippet for AsyncResource and EE integration
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PR-URL: #33751
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
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puzpuzpuz committed Jun 11, 2020
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Showing 1 changed file with 28 additions and 1 deletion.
29 changes: 28 additions & 1 deletion doc/api/async_hooks.md
Original file line number Diff line number Diff line change
Expand Up @@ -509,7 +509,7 @@ createHook({
}
}).enable();

const server = createServer(function(req, res) {
const server = createServer((req, res) => {
executionAsyncResource()[sym] = { state: req.url };
setTimeout(function() {
res.end(JSON.stringify(executionAsyncResource()[sym]));
Expand Down Expand Up @@ -864,6 +864,31 @@ for (let i = 0; i < 10; i++) {
}
```

### Integrating `AsyncResource` with `EventEmitter`

Event listeners triggered by an [`EventEmitter`][] may be run in a different
execution context than the one that was active when `eventEmitter.on()` was
called.

The following example shows how to use the `AsyncResource` class to properly
associate an event listener with the correct execution context. The same
approach can be applied to a [`Stream`][] or a similar event-driven class.

```js
const { createServer } = require('http');
const { AsyncResource, executionAsyncId } = require('async_hooks');

const server = createServer((req, res) => {
const asyncResource = new AsyncResource('request');
// The listener will always run in the execution context of `asyncResource`.
req.on('close', asyncResource.runInAsyncScope.bind(asyncResource, () => {
// Prints: true
console.log(asyncResource.asyncId() === executionAsyncId());
}));
res.end();
}).listen(3000);
```

## Class: `AsyncLocalStorage`
<!-- YAML
added:
Expand Down Expand Up @@ -1116,8 +1141,10 @@ to associate the asynchronous operation with the correct execution context.
[`destroy` callback]: #async_hooks_destroy_asyncid
[`init` callback]: #async_hooks_init_asyncid_type_triggerasyncid_resource
[`promiseResolve` callback]: #async_hooks_promiseresolve_asyncid
[`EventEmitter`]: events.html#events_class_eventemitter
[Hook Callbacks]: #async_hooks_hook_callbacks
[PromiseHooks]: https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit
[`Stream`]: stream.html#stream_stream
[`Worker`]: worker_threads.html#worker_threads_class_worker
[promise execution tracking]: #async_hooks_promise_execution_tracking
[`util.promisify()`]: util.html#util_util_promisify_original

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