Failed to load c++ bson extension, using pure JS version
npm install mongodb
npm install bson
npm install assert
反复执行这三步安装，就不会有上面的问题了。
可用以下代码测试
var MongoClient = require('mongodb').MongoClient;
var assert = require('assert');
var url = 'mongodb://localhost:27017/test';
MongoClient.connect(url, function(err, db) {
assert.equal(null, err);
console.log("Connected correctly to server.");
db.close();
});