發現nodejs噴出了Use emitter.setMaxListeners() to increase limit 的警告，
找到stackoverflow的討論，但也有人說明是不是要把on 用once取代掉或有on被寫在for回圈裡面。
By default, a maximum of 10 listeners can be registered for any single event.
If it's your code, you can specify maxListeners via:
const emitter = new EventEmitter()
emitter.setMaxListeners(100)
// or 0 to turn off the limit
emitter.setMaxListeners(0)
But if it's not your code you can use the trick to increase the default limit globally:
require('events').EventEmitter.prototype._maxListeners = 100;
Of course you can turn off the limits but be careful:
// turn off limits by default (BE CAREFUL)
require('events').EventEmitter.prototype._maxListeners = 0;
BTW. The code should be at the very beginning of the app.
ADD: Since node 0.11 this code also works to change the default limit:
require('events').EventEmitter.defaultMaxListeners = 0
