emitter.once(eventName, listener)

为名为 eventName 的事件添加单次的 listener 函数。 下次触发 eventName 时，将移除此监听器，然后再调用。

• eventName <string> | <symbol> The name of the event.
• listener <Function> The callback function
• Returns: <EventEmitter>

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!');
});

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

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.

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