EventEmitter.on exits function after async iteration of events

[mitsos1os]

• Version:v.12.16.1
• Platform: Linux 5.3.0-42-generic fix LICENSE #34~18.04.1-Ubuntu SMP Fri Feb 28 13:42:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem:

What steps will reproduce the bug?

I am not sure if this is actually an issue or it works as expected, but the newly introduced EventEmiter.on functionality used in a for await, exits the function in which is called after iteration, without running the following commands. Notice the snippet from the actual documentation with an extra log line added:

const { on, EventEmitter } = require('events');

(async () => {
 const ee = new EventEmitter();

 // Emit later on
 process.nextTick(() => {
 ee.emit('foo', 'bar');
 ee.emit('foo', 42);
 });

 for await (const event of on(ee, 'foo')) {
 // The execution of this inner block is synchronous and it
 // processes one event at a time (even with await). Do not use
 // if concurrent execution is required.
 console.log(event); // prints ['bar'] [42]
 }
 console.log('The End'); // This is never printed...!
})();

However this is not the case using a regular AsyncIterator:

(async () => {
 
 async function* asyncGenerator() {
 let i = 0;
 while (i < 3) {
 yield i++;
 }
 }

 for await (const event of asyncGenerator()) {
 // The execution of this inner block is synchronous and it
 // processes one event at a time (even with await). Do not use
 // if concurrent execution is required.
 console.log(event); // prints ['bar'] [42]
 }
 console.log('The End'); // This is printed normally!
})();

So that is why I believe it is a problem with EventEmitter.on functionality...

What is the expected behavior?

I believe it should continue execution of code after the loop

What do you see instead?

Code following loop is not run.

[himself65]

node/lib/events.js

Lines 697 to 700 in ffdf1de

	// Wait until an event happens
	return new Promise(function(resolve, reject) {
	unconsumedPromises.push({ resolve, reject });
	});

return new Promse will make a pause and wait until the process exit

[himself65]

same as

;(async () => {
 await new Promise(() => {})
 console.log('The End') // This is never printed...!
})()

[Jack-Works]

same as

;(async () => {
 await new Promise(() => {})
 console.log('The End') // This is never printed...!
})()

If you read the ee from the closure and emit one new event, the async iterator will advance by one step.

[himself65]

same as

;(async () => {
 await new Promise(() => {})
 console.log('The End') // This is never printed...!
})()

If you read the ee from the closure and emit one new event, the async iterator will advance by one step.

yes, sure. I just told what caused it.

[mitsos1os]

Ok guys, I understand now.
It exits because nothing else is registered in the event loop and not because the iteration of AsyncIterable returned by EventEmitter.onwas over done=true. The iterator is always pending for a new event emission so that a new loop turn in for await is triggered.

My misunderstanding was based on the provided documentation that stated

It removes all listeners when exiting the loop.

I mistook this as if it meant that the loop exits, (so iterator is over), after all events registered up to that time are fired.

Closing the issue as invalid.
Thanks!