Node.js v20.20.6 文档

向监听器传递参数和 this#>

【Passing arguments and this to listeners】

eventEmitter.emit() 方法允许将任意一组参数传递给监听器函数。请注意，当调用普通的监听器函数时，标准的 this 关键字会被故意设置为引用附加了该监听器的 EventEmitter 实例。

【The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(shapeMode)]: false,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(shapeMode)]: false,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');拷贝

可以将 ES6 箭头函数用作监听器，但是这样做时，this 关键字将不再引用 EventEmitter 实例：

【It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b undefined
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');拷贝

异步与同步#>

【Asynchronous vs. synchronous】

EventEmitter 会按照监听器注册的顺序同步调用所有监听器。这确保了事件的正确顺序，并有助于避免竞争条件和逻辑错误。在适当的情况下，监听器函数可以使用 setImmediate() 或 process.nextTick() 方法切换到异步操作模式：

【The EventEmitter calls all listeners synchronously in the order in which they were registered. This ensures the proper sequencing of events and helps avoid race conditions and logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');拷贝

只处理一次事件#>

【Handling events only once】

当使用 eventEmitter.on() 方法注册一个监听器时，每当发出指定事件时，该监听器都会被调用。

【When a listener is registered using the eventEmitter.on() method, that listener is invoked every time the named event is emitted.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2拷贝

使用 eventEmitter.once() 方法，可以注册一个对于特定事件最多只会被调用一次的监听器。一旦事件被触发，监听器将被注销，然后才会被调用。

【Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignoredconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored拷贝

错误事件#>

【Error events】

当 EventEmitter 实例中发生错误时，通常的做法是触发一个 'error' 事件。这些在 Node.js 中被视为特殊情况。

【When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.】

如果一个 EventEmitter 没有为 'error' 事件至少注册一个监听器，而 'error' 事件被触发时，错误会被抛出，打印堆栈跟踪，并且 Node.js 进程会退出。

【If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.jsconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js拷贝

为了防止 Node.js 进程崩溃，可以使用 domain 模块。（但请注意，node:domain 模块已被废弃。）

【To guard against crashing the Node.js process the domain module can be used. (Note, however, that the node:domain module is deprecated.)】

作为最佳实践，应始终为 'error' 事件添加监听器。

【As a best practice, listeners should always be added for the 'error' events.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an errorconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error拷贝

可以通过使用符号 events.errorMonitor 安装监听器来监视 'error' 事件，而无需处理发出的错误。

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

import { EventEmitter, errorMonitor } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.jsconst { EventEmitter, errorMonitor } = require('node:events');

const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js拷贝

捕获对 promise 的拒绝#>

【Capture rejections of promises】

在事件处理程序中使用 async 函数是有问题的，因为在抛出异常时可能会导致未处理的拒绝：

【Using async functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:】

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});const EventEmitter = require('node:events');
const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});拷贝

EventEmitter 构造函数中的 captureRejections 选项或全局设置会改变这种行为，在 Promise 上安装一个 .then(undefined, handler) 处理程序。这个处理程序会将异常异步地路由到 Symbol.for('nodejs.rejection') 方法（如果存在），如果不存在，则路由到 'error' 事件处理程序。

【The captureRejections option in the EventEmitter constructor or the global setting change this behavior, installing a .then(undefined, handler) handler on the Promise. This handler routes the exception asynchronously to the Symbol.for('nodejs.rejection') method if there is one, or to 'error' event handler if there is none.】

import { EventEmitter } from 'node:events';
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;const EventEmitter = require('node:events');
const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;拷贝

将 events.captureRejections = true 设置为 true 将更改所有新的 EventEmitter 实例的默认值。

【Setting events.captureRejections = true will change the default for all new instances of EventEmitter.】

import { EventEmitter } from 'node:events';

EventEmitter.captureRejections = true;
const ee1 = new EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);const events = require('node:events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);拷贝

由 captureRejections 行为生成的 'error' 事件没有捕获处理程序，以避免无限错误循环：建议 不要将 async 函数用作 'error' 事件处理程序。

【The 'error' events that are generated by the captureRejections behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use async functions as 'error' event handlers.】

类：EventEmitter#>

【Class: EventEmitter】

EventEmitter 类由 node:events 模块定义并导出：

【The EventEmitter class is defined and exposed by the node:events module:】

import { EventEmitter } from 'node:events';const EventEmitter = require('node:events');拷贝

所有 EventEmitter 在添加新监听器时都会触发 'newListener' 事件，在移除现有监听器时会触发 'removeListener' 事件。

【All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.】

它支持以下的选项：

【It supports the following option:】

• captureRejections <boolean> 它启用 自动捕获 Promise 拒绝。 默认值: false.

事件：'newListener'#>

【Event: 'newListener'】

• eventName <string> | <symbol> 正在监听的事件名称
• listener <Function> 事件处理函数

EventEmitter 实例将在监听器被添加到其内部监听器数组之前 发出 自身的 'newListener' 事件。

【The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.】

注册了 'newListener' 事件的监听器会接收到事件名称以及正在添加的监听器的引用。

【Listeners registered for the 'newListener' event are passed the event name and a reference to the listener being added.】

事件在添加监听器之前触发这一事实有一个微妙但重要的副作用：在 'newListener' 回调中注册到同一 name 的任何 额外 监听器都会被插入到正在添加的监听器之前。

【The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback are inserted before the listener that is in the process of being added.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   Aconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A拷贝

事件：'removeListener'#>

【Event: 'removeListener'】

• eventName <string> | <symbol> 事件名称
• listener <Function> 事件处理函数

'removeListener' 事件会在 listener 被移除之后触发。

【The 'removeListener' event is emitted after the listener is removed.】

emitter.addListener(eventName, listener)#>

• eventName <string> | <symbol>
• listener <Function>

emitter.on(eventName, listener) 的别名。

【Alias for emitter.on(eventName, listener).】

emitter.emit(eventName[, ...args])#>

• eventName <string> | <symbol>
• ...args <any>
• 返回: <boolean>

同步调用为名为 eventName 的事件注册的每个监听器，按照注册顺序，将提供的参数传递给每个监听器。

【Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.】

如果事件有监听器，则返回 true，否则返回 false。

【Returns true if the event had listeners, false otherwise.】

import { EventEmitter } from 'node: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 listenerconst EventEmitter = require('node: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拷贝

emitter.eventNames()#>

• 返回: <Array>

返回一个数组，列出发出器已注册监听器的事件。数组中的值可以是字符串或 Symbol。

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

import { EventEmitter } from 'node: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) ]const EventEmitter = require('node: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) ]拷贝

emitter.getMaxListeners()#>

• 返回: <integer>

返回 EventEmitter 当前的最大监听器值，该值要么由 emitter.setMaxListeners(n) 设置，要么默认为 events.defaultMaxListeners。

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

emitter.listenerCount(eventName[, listener])#>

• eventName <string> | <symbol> 要监听的事件名称
• listener <Function> 事件处理函数
• 返回值: <integer>

返回正在监听名为 eventName 的事件的监听器数量。如果提供了 listener，则返回该监听器在事件的监听器列表中出现的次数。

【Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.】

emitter.listeners(eventName)#>

• eventName <string> | <symbol>
• 返回值: <Function[]>

返回名为 eventName 的事件的监听器数组的副本。

【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] ] 拷贝

emitter.off(eventName, listener)#>

• eventName <string> | <symbol>
• listener <Function>
• 返回: <EventEmitter>

emitter.removeListener() 的别名。

【Alias for emitter.removeListener().】

emitter.on(eventName, listener)#>

• eventName <string> | <symbol> 事件的名称。
• listener <Function> 回调函数
• 返回: <EventEmitter>

将 listener 函数添加到名为 eventName 的事件的监听器数组末尾。不会检查 listener 是否已被添加。多次传入相同的 eventName 和 listener 组合会导致 listener 被多次添加并调用。

【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 eventName and listener will result in the listener being added, and called, multiple times.】

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

返回 EventEmitter 的引用，以便可以进行链式调用。

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

默认情况下，事件监听器会按照添加的顺序被调用。emitter.prependListener() 方法可作为一种替代方案，将事件监听器添加到监听器数组的开头。

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

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   aconst EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a拷贝

emitter.once(eventName, listener)#>

• eventName <string> | <symbol> 事件的名称。
• listener <Function> 回调函数
• 返回: <EventEmitter>

为名为 eventName 的事件添加一个一次性的 listener 函数。下次触发 eventName 时，该监听器将被移除，然后被调用。

【Adds a one-time listener 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!');
}); 拷贝

返回 EventEmitter 的引用，以便可以进行链式调用。

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

默认情况下，事件监听器会按照添加的顺序被调用。可以使用 emitter.prependOnceListener() 方法作为替代，将事件监听器添加到监听器数组的开头。

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

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   aconst EventEmitter = require('node:events');
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a拷贝

emitter.prependListener(eventName, listener)#>

• eventName <string> | <symbol> 事件的名称。
• listener <Function> 回调函数
• 返回: <EventEmitter>

将 listener 函数添加到名为 eventName 的事件的监听器数组的 开头。不会检查 listener 是否已经被添加。多次传入相同的 eventName 和 listener 组合会导致 listener 被多次添加并多次调用。

【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 eventName and listener will result in the listener being added, and called, multiple times.】

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

返回 EventEmitter 的引用，以便可以进行链式调用。

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

emitter.prependOnceListener(eventName, listener)#>

• eventName <string> | <symbol> 事件的名称。
• listener <Function> 回调函数
• 返回: <EventEmitter>

为名为 eventName 的事件在监听器数组的开头添加一个一次性的 listener 函数。下次触发 eventName 时，这个监听器会被移除，然后执行。

【Adds a one-time listener function for the event named eventName to the beginning 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!');
}); 拷贝

返回 EventEmitter 的引用，以便可以进行链式调用。

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

emitter.removeAllListeners([eventName])#>

• eventName <string> | <symbol>
• 返回值: <EventEmitter>

移除所有监听器，或指定 eventName 的监听器。

【Removes all listeners, or those of the specified eventName.】

移除在代码其他地方添加的监听器是不好的做法，尤其是当 EventEmitter 实例是由其他组件或模块（例如套接字或文件流）创建时。

【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).】

返回 EventEmitter 的引用，以便可以进行链式调用。

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

emitter.removeListener(eventName, listener)#>

• eventName <string> | <symbol>
• listener <Function>
• 返回: <EventEmitter>

从名为 eventName 的事件的监听器数组中移除指定的 listener。

【Removes the specified listener from the listener array for the event named eventName.】

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

removeListener() 最多会从监听器数组中移除一个监听器实例。如果同一个监听器已被多次添加到指定 eventName 的监听器数组中，则必须多次调用 removeListener() 才能移除每一个实例。

一旦事件被触发，当时附加到该事件的所有监听器都会按顺序被调用。这意味着在事件触发之后、最后一个监听器完成执行之前的任何 removeListener() 或 removeAllListeners() 调用，都不会将它们从正在进行的 emit() 中移除。随后的事件行为则如预期一样。

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

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
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:
//   Aconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
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拷贝

由于监听器是通过内部数组管理的，调用此方法会更改在被移除的监听器之后注册的任何监听器的位置索引。这不会影响监听器被调用的顺序，但这意味着通过 emitter.listeners() 方法返回的监听器数组的任何副本都需要重新创建。

【Because listeners are managed using an internal array, calling this will change the position indexes 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.】

当一个函数被多次添加为单个事件的处理程序时（如下面的示例所示），removeListener() 会移除最近添加的实例。在示例中，once('ping') 监听器被移除：

【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:】

import { EventEmitter } from 'node:events';
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');const EventEmitter = require('node:events');
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');拷贝

返回 EventEmitter 的引用，以便可以进行链式调用。

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

emitter.setMaxListeners(n)#>

• n <integer>
• 返回: <EventEmitter>

默认情况下，如果为某个特定事件添加了超过 10 个监听器，EventEmitter 会打印警告。这是一个有用的默认设置，有助于发现内存泄漏。emitter.setMaxListeners() 方法允许修改这个特定 EventEmitter 实例的限制。该值可以设置为 Infinity（或 0），表示监听器数量不受限制。

【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 to Infinity (or 0) to indicate an unlimited number of listeners.】

返回 EventEmitter 的引用，以便可以进行链式调用。

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

emitter.rawListeners(eventName)#>

• eventName <string> | <symbol>
• 返回值: <Function[]>

返回名为 eventName 的事件的监听器数组的副本，包括任何封装器（例如由 .once() 创建的封装器）。

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

import { EventEmitter } from 'node:events';
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');const EventEmitter = require('node:events');
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');拷贝

emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])#>

• err 错误
• eventName <string> | <symbol>
• ...args <any>

Symbol.for('nodejs.rejection') 方法在以下情况下被调用：当在触发事件时发生 Promise 拒绝，并且触发器上启用了 captureRejections。可以使用 events.captureRejectionSymbol 来替代 Symbol.for('nodejs.rejection')。

【The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').】

import { EventEmitter, captureRejectionSymbol } from 'node:events';

class MyClass extends EventEmitter {
  constructor() {
    super({ captureRejections: true });
  }

  [captureRejectionSymbol](err, event, ...args) {
    console.log('rejection happened for', event, 'with', err, ...args);
    this.destroy(err);
  }

  destroy(err) {
    // Tear the resource down here.
  }
}const { EventEmitter, captureRejectionSymbol } = require('node:events');

class MyClass extends EventEmitter {
  constructor() {
    super({ captureRejections: true });
  }

  [captureRejectionSymbol](err, event, ...args) {
    console.log('rejection happened for', event, 'with', err, ...args);
    this.destroy(err);
  }

  destroy(err) {
    // Tear the resource down here.
  }
}拷贝

events.defaultMaxListeners#>

默认情况下，最多可以为任何单个事件注册 10 个监听器。可以使用 emitter.setMaxListeners(n) 方法为单独的 EventEmitter 实例更改此限制。要更改 所有 EventEmitter 实例的默认值，可以使用 events.defaultMaxListeners 属性。如果该值不是正数，将抛出 RangeError。

【By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.】

在设置 events.defaultMaxListeners 时请小心，因为此更改会影响 所有 EventEmitter 实例，包括在更改之前创建的实例。然而，调用 emitter.setMaxListeners(n) 仍然优先于 events.defaultMaxListeners。

【Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.】

这不是一个硬性限制。EventEmitter 实例将允许添加更多监听器，但会在标准错误输出中显示跟踪警告，表明已检测到“可能的 EventEmitter 内存泄漏”。对于任何单个 EventEmitter，可以使用 emitter.getMaxListeners() 和 emitter.setMaxListeners() 方法暂时避免此警告：

【This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:】

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});const EventEmitter = require('node:events');
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});拷贝

可以使用 --trace-warnings 命令行标志来显示此类警告的堆栈跟踪。

【The --trace-warnings command-line flag can be used to display the stack trace for such warnings.】

发出的警告可以使用 process.on('warning') 检查，并且将具有额外的 emitter、type 和 count 属性，分别对应事件触发器实例、事件名称以及附加的监听器数量。它的 name 属性被设置为 'MaxListenersExceededWarning'。

【The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.】

events.errorMonitor#>

此符号用于安装仅监控 'error' 事件的监听器。使用此符号安装的监听器会在常规 'error' 监听器被调用之前被调用。

【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.】

使用此符号安装监听器不会改变 'error' 事件触发后的行为。因此，如果没有安装常规的 'error' 监听器，进程仍然会崩溃。

【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.】

events.getEventListeners(emitterOrTarget, eventName)#>

• emitterOrTarget <EventEmitter> | <EventTarget>
• eventName <string> | <symbol>
• 返回值: <Function[]>

返回名为 eventName 的事件的监听器数组的副本。

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

对于 EventEmitter，这与在触发器上调用 .listeners 的行为完全相同。

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

对于 EventTarget，这是获取该事件目标的事件监听器的唯一方式。这对于调试和诊断非常有用。

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

import { getEventListeners, EventEmitter } from 'node:events';

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

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

events.getMaxListeners(emitterOrTarget)#>

• emitterOrTarget <EventEmitter> | <EventTarget>
• 返回值: <number>

返回当前设置的最大监听器数量。

【Returns the currently set max amount of listeners.】

对于 EventEmitter，这与在触发器上调用 .getMaxListeners 的行为完全相同。

【For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.】

对于 EventTarget 来说，这是获取事件目标最大事件监听器数量的唯一方式。如果单个 EventTarget 上的事件处理程序数量超过了设置的最大值，EventTarget 将打印警告。

【For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.】

import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}const { getMaxListeners, setMaxListeners, EventEmitter } = require('node:events');

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}拷贝

events.once(emitter, name[, options])#>

• emitter <EventEmitter>
• name <string> | <symbol>
• options <Object>
  • signal <AbortSignal> 可用于取消等待事件。
• 返回: <Promise>

创建一个 Promise，当 EventEmitter 发出指定事件时该 Promise 被满足，如果 EventEmitter 在等待期间发出 'error'，则该 Promise 被拒绝。该 Promise 会以一个数组的形式解析，数组内容为发给指定事件的所有参数。

【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.】

此方法故意设计得通用，可用于 web 平台 事件目标 接口，该接口没有特殊的 'error' 事件语义，并且不会监听 'error' 事件。

【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.】

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

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.error('error happened', err);
}const { once, EventEmitter } = require('node: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.error('error happened', err);
  }
}

run();拷贝

对 'error' 事件的特殊处理仅在使用 events.once() 等待其他事件时使用。如果 events.once() 用于等待 'error' 事件本身，那么它将像对待其他任何事件一样处理，没有特殊处理：

【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:】

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

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

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

// Prints: ok boomconst { EventEmitter, once } = require('node:events');

const ee = new EventEmitter();

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

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

// Prints: ok boom拷贝

可以使用 <AbortSignal> 来取消等待该事件：

【An <AbortSignal> can be used to cancel waiting for the event:】

import { EventEmitter, once } from 'node: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(); // Prints: Waiting for the event was canceled!const { EventEmitter, once } = require('node: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(); // Prints: Waiting for the event was canceled!拷贝

等待在 process.nextTick() 上触发的多个事件#>

【Awaiting multiple events emitted on process.nextTick()】

在使用 events.once() 函数来等待同一批 process.nextTick() 操作中触发的多个事件，或在多次事件同步触发时，有一个值得注意的边缘情况。具体来说，由于 process.nextTick() 队列会在 Promise 微任务队列之前被清空，并且由于 EventEmitter 会同步触发所有事件，因此 events.once() 可能会遗漏某些事件。

【There is an edge case worth noting when using the events.once() function to await multiple events emitted on in the same batch of process.nextTick() operations, or whenever multiple events are emitted synchronously. Specifically, because the process.nextTick() queue is drained before the Promise microtask queue, and because EventEmitter emits all events synchronously, it is possible for events.once() to miss an event.】

import { EventEmitter, once } from 'node:events';
import process from 'node:process';

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));拷贝

要同时捕获这两个事件，需要在等待任意一个事件之前先创建每个 Promise，然后就可以使用 Promise.all()、Promise.race() 或 Promise.allSettled()：

【To catch both events, create each of the Promises before awaiting either of them, then it becomes possible to use Promise.all(), Promise.race(), or Promise.allSettled():】

import { EventEmitter, once } from 'node:events';
import process from 'node:process';

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));拷贝

events.captureRejections#>

值：<boolean>

【Value: <boolean>】

更改所有新 EventEmitter 对象的默认 captureRejections 选项。

【Change the default captureRejections option on all new EventEmitter objects.】

events.captureRejectionSymbol#>

值：Symbol.for('nodejs.rejection')

【Value: Symbol.for('nodejs.rejection')】

了解如何编写自定义 拒绝处理程序。

【See how to write a custom rejection handler.】

events.listenerCount(emitter, eventName)#>

• emitter <EventEmitter> 要查询的触发器
• eventName <string> | <symbol> 事件名称

一个类方法，返回在给定 emitter 上注册的指定 eventName 的监听器数量。

【A class method that returns the number of listeners for the given eventName registered on the given emitter.】

import { EventEmitter, listenerCount } from 'node:events';

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

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

events.on(emitter, eventName[, options])#>

• emitter <EventEmitter>
• eventName <string> | <symbol> 要监听的事件名称
• options <Object>
  • signal <AbortSignal> 可用于取消等待的事件。
  • close - <string[]> 将结束迭代的事件名称。
  • highWaterMark - <integer> 默认值: Number.MAX_SAFE_INTEGER 高水位值。每当缓冲的事件数量超过此值时，触发器将被暂停。仅支持实现了 pause() 和 resume() 方法的触发器。
  • lowWaterMark - <integer> 默认值: 1 低水位值。每当缓冲的事件数量低于此值时，触发器将恢复。仅支持实现了 pause() 和 resume() 方法的触发器。
• 返回: <AsyncIterator>，用于迭代 emitter 发出的 eventName 事件

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

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 hereconst { on, EventEmitter } = require('node: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
})();拷贝

返回一个 AsyncIterator，用于迭代 eventName 事件。如果 EventEmitter 发出 'error'，它将抛出异常。退出循环时会移除所有监听器。每次迭代返回的 value 是一个由触发事件的参数组成的数组。

【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.】

可以使用 <AbortSignal> 来取消等待事件：

【An <AbortSignal> can be used to cancel waiting on events:】

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

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());const { on, EventEmitter } = require('node: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());拷贝

events.setMaxListeners(n[, ...eventTargets])#>

• n <number> 一个非负数。每个 EventTarget 事件的最大监听器数量。
• ...eventsTargets <EventTarget[]>|<EventEmitter[]> 零个或多个 <EventTarget> 或 <EventEmitter> 实例。如果未指定任何实例，n 将作为所有新创建的 <EventTarget> 和 <EventEmitter> 对象的默认最大值。

import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);const {
  setMaxListeners,
  EventEmitter,
} = require('node:events');

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);拷贝

events.addAbortListener(signal, listener)#>

• signal <AbortSignal>
• listener <Function> | <EventListener>
• 返回值: <Disposable> 一个可释放的对象，用于移除 abort 监听器。

在提供的 signal 上监听一次 abort 事件。

【Listens once to the abort event on the provided signal.】

监听中止信号（abort）上的 abort 事件是不安全的，可能会导致资源泄漏，因为持有该信号的其他第三方可以调用 e.stopImmediatePropagation()。不幸的是，Node.js 无法更改这一点，因为这会违反 Web 标准。此外，原始 API 也很容易让人忘记移除监听器。

【Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.】

这个 API 允许在 Node.js API 中安全使用 AbortSignal，通过监听事件解决这两个问题，从而 stopImmediatePropagation 不会阻止监听器的运行。

【This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.】

返回一次性内容，以便更轻松地取消订阅。

【Returns a disposable so that it may be unsubscribed from more easily.】

const { addAbortListener } = require('node:events');

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}import { addAbortListener } from 'node:events';

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}拷贝

类：events.EventEmitterAsyncResource 继承自 EventEmitter#>

【Class: events.EventEmitterAsyncResource extends EventEmitter】

将 EventEmitter 与 <AsyncResource> 集成，用于需要手动异步跟踪的 EventEmitter。具体来说，events.EventEmitterAsyncResource 实例发出的所有事件都将在其 异步上下文 中运行。

【Integrates EventEmitter with <AsyncResource> for EventEmitters that require manual async tracking. Specifically, all events emitted by instances of events.EventEmitterAsyncResource will run within its async context.】

import { EventEmitterAsyncResource, EventEmitter } from 'node:events';
import { notStrictEqual, strictEqual } from 'node:assert';
import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
  strictEqual(executionAsyncId(), ee1.asyncId);
  strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});

const ee2 = new EventEmitter();

// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
  notStrictEqual(executionAsyncId(), ee2.asyncId);
  notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});

Promise.resolve().then(() => {
  ee1.emit('foo');
  ee2.emit('foo');
});const { EventEmitterAsyncResource, EventEmitter } = require('node:events');
const { notStrictEqual, strictEqual } = require('node:assert');
const { executionAsyncId, triggerAsyncId } = require('node:async_hooks');

// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
  strictEqual(executionAsyncId(), ee1.asyncId);
  strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});

const ee2 = new EventEmitter();

// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
  notStrictEqual(executionAsyncId(), ee2.asyncId);
  notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});

Promise.resolve().then(() => {
  ee1.emit('foo');
  ee2.emit('foo');
});拷贝

EventEmitterAsyncResource 类具有与 EventEmitter 和 AsyncResource 本身相同的方法，并接受相同的选项。

【The EventEmitterAsyncResource class has the same methods and takes the same options as EventEmitter and AsyncResource themselves.】

new events.EventEmitterAsyncResource([options])#>

• options <Object>
  • captureRejections <boolean> 它启用 自动捕获 Promise 拒绝。 默认值: false。
  • name <string> 异步事件的类型。默认值: new.target.name。
  • triggerAsyncId <number> 创建此异步事件的执行上下文 ID。默认值: executionAsyncId()。
  • requireManualDestroy <boolean> 如果设置为 true，对象被垃圾回收时将禁用 emitDestroy。通常不需要设置（即使手动调用 emitDestroy 也不需要），除非已获取资源的 asyncId 并使用该 ID 调用敏感 API 的 emitDestroy。当设置为 false 时，垃圾回收时的 emitDestroy 调用仅在至少有一个活动的 destroy 钩子存在时才会执行。 默认值: false。

eventemitterasyncresource.asyncId#>

• 类型：<number> 分配给该资源的唯一 asyncId。

eventemitterasyncresource.asyncResource#>

• 类型：基础的 <AsyncResource>。

返回的 AsyncResource 对象具有一个额外的 eventEmitter 属性，用于提供对该 EventEmitterAsyncResource 的引用。

【The returned AsyncResource object has an additional eventEmitter property that provides a reference to this EventEmitterAsyncResource.】

eventemitterasyncresource.emitDestroy()#>

调用所有 destroy 钩子。此方法应该只被调用一次。如果被调用多次，将会抛出错误。此方法 必须 手动调用。如果资源被留给垃圾回收器回收，则 destroy 钩子将永远不会被调用。

【Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.】

eventemitterasyncresource.triggerAsyncId#>

• 类型: <number> 与传递给 AsyncResource 构造函数的 triggerAsyncId 相同。

EventTarget 和 Event API#>

【EventTarget and Event API】

EventTarget 和 Event 对象是 Node.js 特有的 EventTarget Web API 实现，由一些 Node.js 核心 API 提供。

【The EventTarget and Event objects are a Node.js-specific implementation of the EventTarget Web API that are exposed by some Node.js core APIs.】

const target = new EventTarget();

target.addEventListener('foo', (event) => {
  console.log('foo event happened!');
}); 拷贝

Node.js EventTarget 与 DOM EventTarget#>

【Node.js EventTarget vs. DOM EventTarget】

Node.js 的 EventTarget 与 EventTarget Web API 之间有两个主要区别：

【There are two key differences between the Node.js EventTarget and the EventTarget Web API:】

1. 虽然 DOM EventTarget 实例 可能 是层级化的，但在 Node.js 中没有层级和事件传播的概念。也就是说，分派到 EventTarget 的事件不会通过可能各自拥有自己事件处理程序集合的嵌套目标对象层级传播。
2. 在 Node.js 的 EventTarget 中，如果一个事件监听器是异步函数或返回一个 Promise，并且返回的 Promise 被拒绝（reject），则该拒绝会被自动捕获，并以与同步抛出异常的监听器相同的方式处理（详情见 EventTarget 错误处理）。

NodeEventTarget 与 EventEmitter#>

【NodeEventTarget vs. EventEmitter】

NodeEventTarget 对象实现了经过修改的 EventEmitter API 子集，使其在某些情况下可以接近地 模拟 一个 EventEmitter。NodeEventTarget 不是 EventEmitter 的实例，并且在大多数情况下不能替代 EventEmitter 使用。

【The NodeEventTarget object implements a modified subset of the EventEmitter API that allows it to closely emulate an EventEmitter in certain situations. A NodeEventTarget is not an instance of EventEmitter and cannot be used in place of an EventEmitter in most cases.】

1. 与 EventEmitter 不同，每个事件 type 的任何给定 listener 最多只能注册一次。多次尝试注册同一个 listener 将被忽略。
2. NodeEventTarget 并不模拟完整的 EventEmitter API。具体来说，prependListener()、prependOnceListener()、rawListeners() 和 errorMonitor API 未被模拟。'newListener' 和 'removeListener' 事件也不会被触发。
3. NodeEventTarget 对类型为 'error' 的事件不会实现任何特殊的默认行为。
4. NodeEventTarget 支持将 EventListener 对象以及函数用作所有事件类型的处理程序。

事件监听器#>

【Event listener】

注册到某个事件 type 的事件监听器可以是 JavaScript 函数，也可以是拥有 handleEvent 属性（其值为函数）的对象。

【Event listeners registered for an event type may either be JavaScript functions or objects with a handleEvent property whose value is a function.】

在任何情况下，处理函数都会被调用，并传入传递给 eventTarget.dispatchEvent() 函数的 event 参数。

【In either case, the handler function is invoked with the event argument passed to the eventTarget.dispatchEvent() function.】

异步函数可以用作事件监听器。如果异步处理函数被拒绝，该拒绝将被捕获并按照 EventTarget 错误处理 所述进行处理。

【Async functions may be used as event listeners. If an async handler function rejects, the rejection is captured and handled as described in EventTarget error handling.】

一个处理函数抛出的错误不会阻止其他处理函数被调用。

【An error thrown by one handler function does not prevent the other handlers from being invoked.】

句柄函数的返回值会被忽略。

【The return value of a handler function is ignored.】

句柄始终按照其添加的顺序被调用。

【Handlers are always invoked in the order they were added.】

处理函数可能会修改 event 对象。

【Handler functions may mutate the event object.】

function handler1(event) {
  console.log(event.type);  // Prints 'foo'
  event.a = 1;
}

async function handler2(event) {
  console.log(event.type);  // Prints 'foo'
  console.log(event.a);  // Prints 1
}

const handler3 = {
  handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  },
};

const handler4 = {
  async handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  },
};

const target = new EventTarget();

target.addEventListener('foo', handler1);
target.addEventListener('foo', handler2);
target.addEventListener('foo', handler3);
target.addEventListener('foo', handler4, { once: true }); 拷贝

EventTarget 错误处理#>

【EventTarget error handling】

当注册的事件监听器抛出错误（或返回一个被拒绝的 Promise）时，默认情况下，该错误会被视为 process.nextTick() 上的未捕获异常。这意味着 EventTarget 中的未捕获异常默认会终止 Node.js 进程。

【When a registered event listener throws (or returns a Promise that rejects), by default the error is treated as an uncaught exception on process.nextTick(). This means uncaught exceptions in EventTargets will terminate the Node.js process by default.】

在事件监听器中抛出异常 不会 阻止其他注册的处理程序被调用。

【Throwing within an event listener will not stop the other registered handlers from being invoked.】

EventTarget 并不像 EventEmitter 那样对 'error' 类型事件实现任何特殊的默认处理。

【The EventTarget does not implement any special default handling for 'error' type events like EventEmitter.】

当前，错误首先会被转发到 process.on('error') 事件，然后才会到达 process.on('uncaughtException')。这种行为已被弃用，并将在未来的版本中更改，以使 EventTarget 与其他 Node.js API 保持一致。任何依赖 process.on('error') 事件的代码都应与新的行为保持一致。

【Currently errors are first forwarded to the process.on('error') event before reaching process.on('uncaughtException'). This behavior is deprecated and will change in a future release to align EventTarget with other Node.js APIs. Any code relying on the process.on('error') event should be aligned with the new behavior.】

类：Event#>

【Class: Event】

Event 对象是 事件 Web API 的一种改编。实例由 Node.js 内部创建。

【The Event object is an adaptation of the Event Web API. Instances are created internally by Node.js.】

event.bubbles#>

• 类型: <boolean> 总是返回 false。

这在 Node.js 中没有使用，纯粹是为了完整性而提供的。

【This is not used in Node.js and is provided purely for completeness.】

event.cancelBubble#>

• 类型: <boolean>

如果设置为 true，则为 event.stopPropagation() 的别名。在 Node.js 中不使用，仅为完整性而提供。

【Alias for event.stopPropagation() if set to true. This is not used in Node.js and is provided purely for completeness.】

event.cancelable#>

• 类型：<boolean> 如果事件是使用 cancelable 选项创建的，则为真。

event.composed#>

• 类型: <boolean> 总是返回 false。

这在 Node.js 中没有使用，纯粹是为了完整性而提供的。

【This is not used in Node.js and is provided purely for completeness.】

event.composedPath()#>

返回一个数组，其中包含当前 EventTarget 作为唯一条目，如果事件未被分发则返回空数组。这在 Node.js 中不使用，仅为完整性提供。

【Returns an array containing the current EventTarget as the only entry or empty if the event is not being dispatched. This is not used in Node.js and is provided purely for completeness.】

event.currentTarget#>

• 类型：<EventTarget> 触发事件的 EventTarget。

event.target 的别名。

【Alias for event.target.】

event.defaultPrevented#>

• 类型: <boolean>

如果 cancelable 为 true 且已调用 event.preventDefault()，则为 true。

【Is true if cancelable is true and event.preventDefault() has been called.】

event.eventPhase#>

• 类型: <number> 当事件未被分发时返回 0，当事件正在分发时返回 2。

这在 Node.js 中没有使用，纯粹是为了完整性而提供的。

【This is not used in Node.js and is provided purely for completeness.】

event.initEvent(type[, bubbles[, cancelable]])#>

• type <string>
• bubbles <boolean>
• cancelable <boolean>

与事件构造函数重复，并且无法设置 composed。这在 Node.js 中不使用，仅为完整性提供。

【Redundant with event constructors and incapable of setting composed. This is not used in Node.js and is provided purely for completeness.】

event.isTrusted#>

• 类型: <boolean>

<AbortSignal> "abort"事件被触发时，isTrusted 设置为 true。在所有其他情况下，其值为 false。

【The <AbortSignal> "abort" event is emitted with isTrusted set to true. The value is false in all other cases.】

event.preventDefault()#>

如果 cancelable 为 true，则将 defaultPrevented 属性设置为 true。

【Sets the defaultPrevented property to true if cancelable is true.】

event.returnValue#>

• 类型: <boolean> 如果事件未被取消，则为真。

event.returnValue 的值总是与 event.defaultPrevented 相反。 这在 Node.js 中不使用，仅为了完整性而提供。

【The value of event.returnValue is always the opposite of event.defaultPrevented. This is not used in Node.js and is provided purely for completeness.】

event.srcElement#>

• 类型：<EventTarget> 触发事件的 EventTarget。

event.target 的别名。

【Alias for event.target.】

event.stopImmediatePropagation()#>

当前事件监听器完成后停止调用。

【Stops the invocation of event listeners after the current one completes.】

event.stopPropagation()#>

这在 Node.js 中没有使用，纯粹是为了完整性而提供的。

【This is not used in Node.js and is provided purely for completeness.】

event.target#>

• 类型：<EventTarget> 触发事件的 EventTarget。

event.timeStamp#>

• 类型: <number>

Event 对象创建时的毫秒时间戳。

【The millisecond timestamp when the Event object was created.】

event.type#>

• 类型: <string>

事件类型标识符。

【The event type identifier.】

类：EventTarget#>

【Class: EventTarget】

eventTarget.addEventListener(type, listener[, options])#>

• type <string>
• listener <Function> | <EventListener>
• options <Object>
  • once <boolean> 当值为 true 时，监听器在第一次调用后会自动移除。默认值： false。
  • passive <boolean> 当值为 true 时，表示监听器不会调用 Event 对象的 preventDefault() 方法的提示。默认值： false。
  • capture <boolean> Node.js 不直接使用。为 API 完整性而添加。默认值： false。
  • signal <AbortSignal> 当调用给定的 AbortSignal 对象的 abort() 方法时，监听器将被移除。

为 type 事件添加新的处理程序。每个给定的 listener 在每个 type 和每个 capture 选项值下仅会被添加一次。

【Adds a new handler for the type event. Any given listener is added only once per type and per capture option value.】

如果 once 选项为 true，listener 会在下一次触发 type 事件后被移除。

【If the once option is true, the listener is removed after the next time a type event is dispatched.】

capture 选项在 Node.js 中除了按照 EventTarget 规范跟踪已注册的事件监听器外，不会以任何功能方式使用。具体来说，注册 listener 时会将 capture 选项用作键的一部分。每个单独的 listener 可以在 capture = false 的情况下添加一次，也可以在 capture = true 的情况下添加一次。

【The capture option is not used by Node.js in any functional way other than tracking registered event listeners per the EventTarget specification. Specifically, the capture option is used as part of the key when registering a listener. Any individual listener may be added once with capture = false, and once with capture = true.】

function handler(event) {}

const target = new EventTarget();
target.addEventListener('foo', handler, { capture: true });  // first
target.addEventListener('foo', handler, { capture: false }); // second

// Removes the second instance of handler
target.removeEventListener('foo', handler);

// Removes the first instance of handler
target.removeEventListener('foo', handler, { capture: true }); 拷贝

eventTarget.dispatchEvent(event)#>

• event <Event>
• 返回：<boolean> 如果事件的 cancelable 属性值为 false 或其 preventDefault() 方法未被调用，则返回 true，否则返回 false。

将 event 分派给 event.type 的处理程序列表。

【Dispatches the event to the list of handlers for event.type.】

注册的事件监听器会按注册顺序同步调用。

【The registered event listeners is synchronously invoked in the order they were registered.】

eventTarget.removeEventListener(type, listener[, options])#>

• type <string>
• listener <Function> | <EventListener>
• options <Object>
  • capture <boolean>

从事件 type 的处理程序列表中移除 listener。

【Removes the listener from the list of handlers for event type.】

类：CustomEvent#>

【Class: CustomEvent】

• 继承: <Event>

CustomEvent 对象是 CustomEvent Web API 的一种改编。实例由 Node.js 内部创建。

【The CustomEvent object is an adaptation of the CustomEvent Web API. Instances are created internally by Node.js.】

event.detail#>

• 类型: <any> 返回初始化时传入的自定义数据。

只读。

【Read-only.】

类：NodeEventTarget#>

【Class: NodeEventTarget】

• 继承: <EventTarget>

NodeEventTarget 是 Node.js 特有的 EventTarget 扩展，用于模拟 EventEmitter API 的一部分功能。

【The NodeEventTarget is a Node.js-specific extension to EventTarget that emulates a subset of the EventEmitter API.】

nodeEventTarget.addListener(type, listener)#>

• type <string>

• listener <Function> | <EventListener>

• 返回: <EventTarget> 这个

Node.js 特定的 EventTarget 类扩展，它模拟了等效的 EventEmitter API。addListener() 和 addEventListener() 之间唯一的区别是 addListener() 会返回 EventTarget 的引用。

【Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.】

nodeEventTarget.emit(type, arg)#>

• type <string>
• arg <any>
• 返回值: <boolean> 如果已为 type 注册了事件监听器则返回 true，否则返回 false。

Node.js 特定的 EventTarget 类扩展，会将 arg 分发到 type 的处理程序列表中。

【Node.js-specific extension to the EventTarget class that dispatches the arg to the list of handlers for type.】

nodeEventTarget.eventNames()#>

• 返回值: <string[]>

Node.js 特定的 EventTarget 类扩展，返回已注册事件监听器的事件 type 名称数组。

【Node.js-specific extension to the EventTarget class that returns an array of event type names for which event listeners are registered.】

nodeEventTarget.listenerCount(type)#>

• type <string>

• 返回: <number>

Node.js 专用的 EventTarget 类扩展，用于返回注册在指定 type 的事件监听器数量。

【Node.js-specific extension to the EventTarget class that returns the number of event listeners registered for the type.】

nodeEventTarget.setMaxListeners(n)#>

• n <number>

Node.js 专用的 EventTarget 类扩展，用于将最大事件监听器数量设置为 n。

【Node.js-specific extension to the EventTarget class that sets the number of max event listeners as n.】

nodeEventTarget.getMaxListeners()#>

• 返回: <number>

Node.js 特定的 EventTarget 类扩展，用于返回最大事件监听器的数量。

【Node.js-specific extension to the EventTarget class that returns the number of max event listeners.】

nodeEventTarget.off(type, listener[, options])#>

• type <string>

• listener <Function> | <EventListener>

• options <Object>

  • capture <boolean>

• 返回: <EventTarget> 这个

Node.js 特定别名，用于 eventTarget.removeEventListener()。

【Node.js-specific alias for eventTarget.removeEventListener().】

nodeEventTarget.on(type, listener)#>

• type <string>

• listener <Function> | <EventListener>

• 返回: <EventTarget> 这个

Node.js 特定的 eventTarget.addEventListener() 别名。

【Node.js-specific alias for eventTarget.addEventListener().】

nodeEventTarget.once(type, listener)#>

• type <string>

• listener <Function> | <EventListener>

• 返回: <EventTarget> 这个

Node.js 特定的 EventTarget 类扩展，为给定的事件 type 添加一个 once 监听器。这相当于调用 on 并将 once 选项设置为 true。

【Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.】

nodeEventTarget.removeAllListeners([type])#>

• type <string>

• 返回: <EventTarget> 这个

Node.js 特定的 EventTarget 类扩展。如果指定了 type，则移除该类型的所有已注册监听器，否则移除所有已注册的监听器。

【Node.js-specific extension to the EventTarget class. If type is specified, removes all registered listeners for type, otherwise removes all registered listeners.】

nodeEventTarget.removeListener(type, listener[, options])#>

• type <string>

• listener <Function> | <EventListener>

• options <Object>

  • capture <boolean>

• 返回: <EventTarget> 这个

Node.js 特定的 EventTarget 类扩展，用于移除给定 type 的 listener。removeListener() 和 removeEventListener() 之间唯一的区别是，removeListener() 会返回一个 EventTarget 的引用。

【Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.】