Koa.md

nextjs-koa-api / Koa

Class: Koa<StateT, ContextT>

Type parameters

Name	Type
StateT	DefaultState
ContextT	DefaultContext

Hierarchy

• EventEmitter

  ↳ Koa

  ↳↳ KoaApi

Table of contents

Constructors

• constructor

Properties

• context
• env
• keys
• maxIpsCount
• middleware
• proxy
• proxyIpHeader
• request
• response
• silent
• subdomainOffset
• captureRejectionSymbol
• captureRejections
• defaultMaxListeners
• errorMonitor

Methods

• addListener
• callback
• createContext
• emit
• eventNames
• getMaxListeners
• inspect
• listen
• listenerCount
• listeners
• off
• on
• once
• onerror
• prependListener
• prependOnceListener
• rawListeners
• removeAllListeners
• removeListener
• setMaxListeners
• toJSON
• use
• getEventListeners
• listenerCount
• on
• once
• setMaxListeners

Constructors

constructor

• new Koa<StateT, ContextT>(options?)

Type parameters

Name	Type
StateT	DefaultState
ContextT	DefaultContext

Parameters

Name	Type
options?	Object
options.env?	string
options.keys?	string[]
options.maxIpsCount?	number
options.proxy?	boolean
options.proxyIpHeader?	string
options.subdomainOffset?	number

Overrides

EventEmitter.constructor

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:471

Properties

context

• context: BaseContext & ContextT

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:454

---

env

• env: string

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:453

---

keys

• keys: string[] | Keygrip

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:458

---

maxIpsCount

• maxIpsCount: number

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:450

---

middleware

• middleware: Middleware<StateT, ContextT, any>[]

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:451

---

proxy

• proxy: boolean

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:448

---

proxyIpHeader

• proxyIpHeader: string

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:449

---

request

• request: BaseRequest

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:455

---

response

• response: BaseResponse

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:456

---

silent

• silent: boolean

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:457

---

subdomainOffset

• subdomainOffset: number

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:452

---

captureRejectionSymbol

▪ Static Readonly captureRejectionSymbol: typeof captureRejectionSymbol

Inherited from

EventEmitter.captureRejectionSymbol

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:301

---

captureRejections

▪ Static captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

Inherited from

EventEmitter.captureRejections

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:306

---

defaultMaxListeners

▪ Static defaultMaxListeners: number

Inherited from

EventEmitter.defaultMaxListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:307

---

errorMonitor

▪ Static Readonly errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted, therefore the process will still crash if no regular 'error' listener is installed.

Inherited from

EventEmitter.errorMonitor

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:300

Methods

addListener

▸ addListener(eventName, listener): Koa

Alias for emitter.on(eventName, listener).

since v0.1.26

Parameters

Name	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

Koa

Inherited from

EventEmitter.addListener

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:327

---

callback

▸ callback(): (req: IncomingMessage | Http2ServerRequest, res: ServerResponse | Http2ServerResponse) => void

Return a request handler callback for node's native http/http2 server.

Returns

fn

▸ (req, res): void

Return a request handler callback for node's native http/http2 server.

Parameters

Name	Type
req	IncomingMessage | Http2ServerRequest
res	ServerResponse | Http2ServerResponse

Returns

void

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:520

---

createContext

▸ createContext<StateT>(req, res): ParameterizedContext<StateT, DefaultContext, unknown>

Initialize a new context.

api private

Type parameters

Name	Type
StateT	DefaultState

Parameters

Name	Type
req	IncomingMessage
res	ServerResponse

Returns

ParameterizedContext<StateT, DefaultContext, unknown>

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:527

---

emit

▸ emit(eventName, ...args): boolean

Synchronously calls each of the listeners registered for the event namedeventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

const EventEmitter = require('events')
const myEmitter = new EventEmitter()

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener')
})
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`)
})
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ')
  console.log(`event with parameters ${parameters} in third listener`)
})

console.log(myEmitter.listeners('event'))

myEmitter.emit('event', 1, 2, 3, 4, 5)

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener

since v0.1.26

Parameters

Name	Type
eventName	string | symbol
...args	any[]

Returns

boolean

Inherited from

EventEmitter.emit

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:583

---

eventNames

▸ eventNames(): (string | symbol)[]

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

const EventEmitter = require('events')
const myEE = new EventEmitter()
myEE.on('foo', () => {})
myEE.on('bar', () => {})

const sym = Symbol('symbol')
myEE.on(sym, () => {})

console.log(myEE.eventNames())
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

since v6.0.0

Returns

(string | symbol)[]

Inherited from

EventEmitter.eventNames

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:642

---

getMaxListeners

▸ getMaxListeners(): number

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.

since v1.0.0

Returns

number

Inherited from

EventEmitter.getMaxListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:499

---

inspect

▸ inspect(): any

Return JSON representation. We only bother showing settings.

Returns

any

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:499

---

listen

▸ listen(port?, hostname?, backlog?, listeningListener?): Server

Shorthand for:

http.createServer(app.callback()).listen(...)

Parameters

Name	Type
port?	number
hostname?	string
backlog?	number
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:485

▸ listen(port, hostname?, listeningListener?): Server

Parameters

Name	Type
port	number
hostname?	string
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:486

▸ listen(port, backlog?, listeningListener?): Server

Parameters

Name	Type
port	number
backlog?	number
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:487

▸ listen(port, listeningListener?): Server

Parameters

Name	Type
port	number
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:488

▸ listen(path, backlog?, listeningListener?): Server

Parameters

Name	Type
path	string
backlog?	number
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:489

▸ listen(path, listeningListener?): Server

Parameters

Name	Type
path	string
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:490

▸ listen(options, listeningListener?): Server

Parameters

Name	Type
options	ListenOptions
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:491

▸ listen(handle, backlog?, listeningListener?): Server

Parameters

Name	Type
handle	any
backlog?	number
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:492

▸ listen(handle, listeningListener?): Server

Parameters

Name	Type
handle	any
listeningListener?	() => void

Returns

Server

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:493

---

listenerCount

▸ listenerCount(eventName): number

Returns the number of listeners listening to the event named eventName.

since v3.2.0

Parameters

Name	Type	Description
eventName	string | symbol	The name of the event being listened for

Returns

number

Inherited from

EventEmitter.listenerCount

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:589

---

listeners

▸ listeners(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!')
})
console.log(util.inspect(server.listeners('connection')))
// Prints: [ [Function] ]

since v0.1.26

Parameters

Name	Type
eventName	string | symbol

Returns

Function[]

Inherited from

EventEmitter.listeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:512

---

off

▸ off(eventName, listener): Koa

Alias for emitter.removeListener().

since v10.0.0

Parameters

Name	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

Koa

Inherited from

EventEmitter.off

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:472

---

on

▸ on(eventName, listener): Koa

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventNameand listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!')
})

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter()
myEE.on('foo', () => console.log('a'))
myEE.prependListener('foo', () => console.log('b'))
myEE.emit('foo')
// Prints:
//   b
//   a

since v0.1.101

Parameters

Name	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

Koa

Inherited from

EventEmitter.on

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:358

---

once

▸ once(eventName, listener): Koa

Adds a one-timelistener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!')
})

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter()
myEE.once('foo', () => console.log('a'))
myEE.prependOnceListener('foo', () => console.log('b'))
myEE.emit('foo')
// Prints:
//   b
//   a

since v0.3.0

Parameters

Name	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

Koa

Inherited from

EventEmitter.once

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:387

---

onerror

▸ onerror(err): void

Default error handler.

api private

Parameters

Name	Type
err	Error

Returns

void

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:537

---

prependListener

▸ prependListener(eventName, listener): Koa

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventNameand listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!')
})

Returns a reference to the EventEmitter, so that calls can be chained.

since v6.0.0

Parameters

Name	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

Koa

Inherited from

EventEmitter.prependListener

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:607

---

prependOnceListener

▸ prependOnceListener(eventName, listener): Koa

Adds a one-timelistener function for the event named eventName to thebeginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!')
})

Returns a reference to the EventEmitter, so that calls can be chained.

since v6.0.0

Parameters

Name	Type	Description
eventName	string | symbol	The name of the event.
listener	(...args: any[]) => void	The callback function

Returns

Koa

Inherited from

EventEmitter.prependOnceListener

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:623

---

rawListeners

▸ rawListeners(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

const emitter = new EventEmitter()
emitter.once('log', () => console.log('log once'))

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log')
const logFnWrapper = listeners[0]

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener()

// Logs "log once" to the console and removes the listener
logFnWrapper()

emitter.on('log', () => console.log('log persistently'))
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log')

// Logs "log persistently" twice
newListeners[0]()
emitter.emit('log')

since v9.4.0

Parameters

Name	Type
eventName	string | symbol

Returns

Function[]

Inherited from

EventEmitter.rawListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:542

---

removeAllListeners

▸ removeAllListeners(event?): Koa

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

since v0.1.26

Parameters

Name	Type
event?	string | symbol

Returns

Koa

Inherited from

EventEmitter.removeAllListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:483

---

removeListener

▸ removeListener(eventName, listener): Koa

Removes the specified listener from the listener array for the event namedeventName.

const callback = (stream) => {
  console.log('someone connected!')
}
server.on('connection', callback)
// ...
server.removeListener('connection', callback)

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that anyremoveListener() or removeAllListeners() calls after emitting andbefore the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

const myEmitter = new MyEmitter()

const callbackA = () => {
  console.log('A')
  myEmitter.removeListener('event', callbackB)
}

const callbackB = () => {
  console.log('B')
}

myEmitter.on('event', callbackA)

myEmitter.on('event', callbackB)

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event')
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event')
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping')listener is removed:

const ee = new EventEmitter()

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

ee.on('ping', pong)
ee.once('ping', pong)
ee.removeListener('ping', pong)

ee.emit('ping')
ee.emit('ping')

Returns a reference to the EventEmitter, so that calls can be chained.

since v0.1.26

Parameters

Name	Type
eventName	string | symbol
listener	(...args: any[]) => void

Returns

Koa

Inherited from

EventEmitter.removeListener

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:467

---

setMaxListeners

▸ setMaxListeners(n): Koa

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

since v0.3.5

Parameters

Name	Type
n	number

Returns

Koa

Inherited from

EventEmitter.setMaxListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:493

---

toJSON

▸ toJSON(): any

Return JSON representation. We only bother showing settings.

Returns

any

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:505

---

use

▸ use<NewStateT, NewContextT>(middleware): Koa

Use the given middleware fn.

Old-style middleware will be converted.

Type parameters

Name	Type
NewStateT	{}
NewContextT	{}

Parameters

Name	Type
middleware	Middleware<StateT & NewStateT, ContextT & NewContextT, any>

Returns

Koa

Defined in

node_modules/.pnpm/@types+koa@2.13.4/node_modules/@types/koa/index.d.ts:512

---

getEventListeners

▸ Static getEventListeners(emitter, name): Function[]

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

const { getEventListeners, EventEmitter } = require('events')

{
  const ee = new EventEmitter()
  const listener = () => console.log('Events are fun')
  ee.on('foo', listener)
  getEventListeners(ee, 'foo') // [listener]
}
{
  const et = new EventTarget()
  const listener = () => console.log('Events are fun')
  et.addEventListener('foo', listener)
  getEventListeners(et, 'foo') // [listener]
}

since v15.2.0, v14.17.0

Parameters

Name	Type
emitter	EventEmitter | DOMEventTarget
name	string | symbol

Returns

Function[]

Inherited from

EventEmitter.getEventListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:270

---

listenerCount

▸ Static listenerCount(emitter, eventName): number

A class method that returns the number of listeners for the given eventNameregistered on the given emitter.

const { EventEmitter, listenerCount } = require('events')
const myEmitter = new EventEmitter()
myEmitter.on('event', () => {})
myEmitter.on('event', () => {})
console.log(listenerCount(myEmitter, 'event'))
// Prints: 2

since v0.9.12

deprecated Since v3.2.0 - Use listenerCount instead.

Parameters

Name	Type	Description
emitter	EventEmitter	The emitter to query
eventName	string | symbol	The event name

Returns

number

Inherited from

EventEmitter.listenerCount

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:242

---

on

▸ Static on(emitter, eventName, options?): AsyncIterableIterator<any>

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]
  }
  // Unreachable here
})()

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

const { on, EventEmitter } = require('events')
const ac = new AbortController()

;(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', { signal: ac.signal })) {
    // 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]
  }
  // Unreachable here
})()

process.nextTick(() => ac.abort())

since v13.6.0, v12.16.0

Parameters

Name	Type	Description
emitter	EventEmitter	-
eventName	string	The name of the event being listened for
options?	StaticEventEmitterOptions	-

Returns

AsyncIterableIterator<any>

that iterates eventName events emitted by the emitter

Inherited from

EventEmitter.on

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:221

---

once

▸ Static once(emitter, eventName, options?): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

const { once, EventEmitter } = require('events')

async function run() {
  const ee = new EventEmitter()

  process.nextTick(() => {
    ee.emit('myevent', 42)
  })

  const [value] = await once(ee, 'myevent')
  console.log(value)

  const err = new Error('kaboom')
  process.nextTick(() => {
    ee.emit('error', err)
  })

  try {
    await once(ee, 'myevent')
  } catch (err) {
    console.log('error happened', err)
  }
}

run()

The special handling of the 'error' event is only used when events.once()is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

const { EventEmitter, once } = require('events')

const ee = new EventEmitter()

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.log('error', err.message))

ee.emit('error', new Error('boom'))

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

const { EventEmitter, once } = require('events')

const ee = new EventEmitter()
const ac = new AbortController()

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal })
    console.log('event emitted!')
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!')
    } else {
      console.error('There was an error', error.message)
    }
  }
}

foo(ee, 'foo', ac.signal)
ac.abort() // Abort waiting for the event
ee.emit('foo') // Prints: Waiting for the event was canceled!

since v11.13.0, v10.16.0

Parameters

Name	Type
emitter	NodeEventTarget
eventName	string | symbol
options?	StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited from

EventEmitter.once

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:157

▸ Static once(emitter, eventName, options?): Promise<any[]>

Parameters

Name	Type
emitter	DOMEventTarget
eventName	string
options?	StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited from

EventEmitter.once

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:162

---

setMaxListeners

▸ Static setMaxListeners(n?, ...eventTargets): void

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The EventEmitter.setMaxListeners() method allows the default limit to be modified (if eventTargets is empty) or modify the limit specified in every EventTarget | EventEmitter passed as arguments. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

EventEmitter.setMaxListeners(20)
// Equivalent to
EventEmitter.defaultMaxListeners = 20

const eventTarget = new EventTarget()
// Only way to increase limit for `EventTarget` instances
// as these doesn't expose its own `setMaxListeners` method
EventEmitter.setMaxListeners(20, eventTarget)

since v15.3.0, v14.17.0

Parameters

Name	Type
n?	number
...eventTargets	(EventEmitter | DOMEventTarget)[]

Returns

void

Inherited from

EventEmitter.setMaxListeners

Defined in

node_modules/.pnpm/@types+node@17.0.31/node_modules/@types/node/events.d.ts:290