events: Loop forwards to find listener in removeListener function

removeListener() in events module loop backwards to find listener from array. Consider the following code:

const EventEmitter = require('events')

function pong () {
  console.log('pong')
}

let a = new EventEmitter()

a.once('ping', pong)
a.on('ping', pong)
a.removeListener('ping', pong)
a.emit('ping')
a.emit('ping')

// output:
// pong

let b = new EventEmitter()

b.on('ping', pong)
b.once('ping', pong)
b.removeListener('ping', pong)
b.emit('ping')
b.emit('ping')

// output:
// pong
// pong

The output shows that for multiple same listeners, the last added one will be the first to remove (LIFO). I think maybe a "FIFO" logic is better for this case. And I find the commit on 5 Mar 2013 which implements "loop backwards" logic.
3e64b56#diff-71dcd327d0ca067b490b22d677f81966
The commit message shows this logic optimize removeAllListeners().But it did effect this specific case. Maybe removeAllListeners() should keep "LIFO" logic and the backward loop optimizing, and removeListener() could use a forward loop.

I think maybe a "FIFO" logic is better for this case.

Can you explain why you think so?

To remove a listener we need to "find" it. Like array.find() I consider this "find action" is from start to end at first. At least document doesn't indicate current "LIFO" logic in removeListener(), it could be confused when someone meet the same case like my example. Of course it only make a difference if you add multiple same listeners with on() and once() at the same time. Perhaps we could update the document to point it out.

At this point, I'd say the easier thing to do is to update the documentation rather than making a semver-major change.

doc: add note about removeListener order

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events: Loop forwards to find listener in removeListener function #21635

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events: Loop forwards to find listener in removeListener function #21635

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TimothyGu commented on Jul 3, 2018

pandaGao commented on Jul 4, 2018

TimothyGu commented on Jul 4, 2018

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