EventEmitter 需要注意的三个地方

eventEmitter.emit 是同步回调的！

例子：

const EventEmitter = require('events');
const emitter = new EventEmitter();

emitter.on('event', () => {
    console.log('an event occurred!');
});
emitter.emit('event');
console.log('hello');

输出：

an event occurred!
hello

emitter.emit(eventName[, arg1][, arg2][, …])#

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

Returns true if the event had listeners, false otherwise.

详见 https://nodejs.org/api/events.html#events_asynchronous_vs_synchronous

emit(‘error’) 是一个特殊事件

详见 https://nodejs.org/api/events.html#events_error_events

当没有设置 error 的事件监听，emit('error') 会抛出 (throw) 新的错误。

当没有设置 ‘error’ 事件监听时，throw new Error() 和 throw 'a string' 有细微的差别

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.emit('error', 'a string');

输出

events.js:159
      throw err;
      ^

Error: Uncaught, unspecified "error" event. (a string)
    at EventEmitter.emit (events.js:157:17)
    at Object.<anonymous> (<my-path>:3:9)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:146:18)
    at node.js:404:3

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.emit('error', new Error('Ouch!!'));

输出

events.js:154
      throw er; // Unhandled 'error' event
      ^

Error: Ouch!!
    at Object.<anonymous> (<my-path>:3:23)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:146:18)
    at node.js:404:3

因为 node 源码就是这么实现的。

if (doError) {
er = arguments[1];
if (domain) {
  if (!er)
    er = new Error('Uncaught, unspecified "error" event');
  er.domainEmitter = this;
  er.domain = domain;
  er.domainThrown = false;
  domain.emit('error', er);
} else if (er instanceof Error) {
  throw er; // Unhandled 'error' event
} else {
  // At least give some kind of context to the user
  var err = new Error('Uncaught, unspecified "error" event. (' + er + ')');
  err.context = er;
  throw err;
}
return false;
}

这会导致开发者难以发现 emit('error') 没有对应监听器的问题。