mongoose listCollections()不返回集合名称，而是返回一个promise pending

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true });

const db = mongoose.connection;

db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {
  // 获取数据库中的所有集合
  db.db.listCollections().toArray(function(err, collections) {
    if (err) {
      console.log(err);
    } else {
      // 遍历集合数组，输出集合名称
      collections.forEach(function(collection) {
        console.log(collection.name);
      });
    }
    db.close();
  });
});