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events: allow monitoring error events #30932

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Flarna wants to merge 3 commits into from
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events: allow monitoring error events #30932

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0a773db
events: allow monitoring error events
Flarna Dec 12, 2019
d156268
rename symbol
Flarna Dec 13, 2019
793205c
fix review findings
Flarna Dec 13, 2019
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@Flarna
Flarna committed Dec 13, 2019
commit d1562689fe8240d763190ec2a1453c161b6256f7
6 changes: 3 additions & 3 deletions doc/api/events.md
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Expand Up @@ -156,11 +156,11 @@ myEmitter.emit('error', new Error('whoops!'));
```

It is possible to monitor `'error'` events without consuming the emitted error
by installing a listener using the symbol `errorMonitorSymbol`.
by installing a listener using the symbol `errorMonitor`.

```js
const myEmitter = new MyEmitter();
myEmitter.on(errorMonitorSymbol, (err) => {
myEmitter.on(errorMonitor, (err) => {
MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
Expand Down Expand Up @@ -360,7 +360,7 @@ the event emitter instance, the event’s name and the number of attached
listeners, respectively.
Its `name` property is set to `'MaxListenersExceededWarning'`.

### EventEmitter.errorMonitorSymbol
### EventEmitter.errorMonitor
<!-- YAML
added: REPLACEME
-->
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10 changes: 5 additions & 5 deletions lib/events.js
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Expand Up @@ -54,7 +54,7 @@ const {
} = require('internal/util/inspect');

const kCapture = Symbol('kCapture');
const kErrorMonitorSymbol = Symbol('events.error-monitor');
const kErrorMonitor = Symbol('events.errorMonitor');

function EventEmitter(opts) {
EventEmitter.init.call(this, opts);
Expand Down Expand Up @@ -83,8 +83,8 @@ ObjectDefineProperty(EventEmitter, 'captureRejections', {
enumerable: true
});

ObjectDefineProperty(EventEmitter, 'errorMonitorSymbol', {
value: kErrorMonitorSymbol,
ObjectDefineProperty(EventEmitter, 'errorMonitor', {
value: kErrorMonitor,
writable: false,
configurable: true,
enumerable: true
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@ljharb ljharb Feb 14, 2020

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by adding an enumerable but nonwritable field, this is a potential breaking change - see googleapis/nodejs-spanner#830 (comment).

In the future, we may want to consider enumerable fields as nonmajor only when they're normal data properties.

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@Flarna Flarna Feb 16, 2020

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Should we change this to a normal property? I used non writable simply because a write to this is usually a bug. I haven't seen any reasonable usecase to change this value but kept it configurable to allow any sort of hack if really needed.

In principle also a normal property added could be seen a breaking if some user added a property with the same name already before in it's code.

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@ljharb ljharb Feb 16, 2020

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Making it non-enumerable, or making it an accessor instead of a data property, would work as well.

Expand Down Expand Up @@ -264,8 +264,8 @@ EventEmitter.prototype.emit = function emit(type, ...args) {

const events = this._events;
if (events !== undefined) {
if (doError && events[kErrorMonitorSymbol] !== undefined)
this.emit(kErrorMonitorSymbol, ...args);
if (doError && events[kErrorMonitor] !== undefined)
this.emit(kErrorMonitor, ...args);
doError = (doError && events.error === undefined);
} else if (!doError)
return false;
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2 changes: 1 addition & 1 deletion test/parallel/test-event-emitter-error-monitor.js
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Expand Up @@ -7,7 +7,7 @@ const EE = new EventEmitter();
const theErr = new Error('MyError');

EE.on(
EventEmitter.errorMonitorSymbol,
EventEmitter.errorMonitor,
common.mustCall((e) => assert.strictEqual(e, theErr), 3)
);

Expand Down