mapnik pool sample code fail

[Cường Văn]

Hello everyone, 

I try to run mapnik pool from node-mapnik-sample, I've already installed generic-pool via command: npm install generic-pool, the code run well and server started, but when I try to access it, it shows an error: 'Class constructors cannot be invoked without 'new'', log file said that error relate to line: this.pools[id] = pool({, this is line 15 in https://raw.githubusercontent.com/mapnik/node-mapnik-sample-code/master/utils/pool.js. This code sample is 4 years ago, so I think it is out of date, anyone know how to solve this problem and implement mapnik pool.

The instruction of mapnik github is quite superficial in my opinion. 

Thank you in advance.