在Node.js中的事件处理程序之前调用事件发射器

Question

I am going through this code and I have seen the nodejs event emitter being called before the event handler was defined( this line and this line ).

When I try to replicate the same, no event gets handled.

    eventEmitter.emit('event');

    eventEmitter.on('event', () => {
        console.log('event handled');
    });

When you put the event emitter after the event handler ' event handled ' gets logged.

Is the code on GitHub wrong? Or does it still work because it is in a module and there is a way the module is imported that allows it to work? Kindly explain. Thank you.

Answer 1

The event emission on line 44 is inside of the handler being bound to the ioChat's "connection" event which is not invoked before the binding of eventEmitter's "get-all-users" event on line 181. So even though the line where the event is invoked comes first, it will not be invoked first.

Example:

eventEmitter.on('connection', () => {
  console.log('this should log second');
  eventEmitter.emit('someEvent');
});

eventEmitter.on('someEvent', () => {
  console.log('this should log third');
});

console.log('this should log first');
eventEmitter.emit('connection');