当esModuleInterop: true或在tsconfig.json中未设置时，使用默认导入import mongoose from 'mongoose'。
当esModuleInterop: false或在tsconfig.json中未设置时，使用命名空间导入import * as mongoose from 'mongoose'。

import mongoose from 'mongoose';
import { MongoMemoryServer } from 'mongodb-memory-server';

let con: mongoose.Connection;
let mongoServer: MongoMemoryServer;

export const connectDB = async () => {
  mongoServer = await MongoMemoryServer.create();
  await mongoose.connect(mongoServer.getUri(), {})
    .catch((err) => {
      console.log(err);
    });
  con = mongoose.connection;
  console.log('con.readyState: ', con.readyState);
}

connectDB();

日志：

⚡  npx ts-node ./index.ts
con.readyState:  1

软件包版本：

"mongodb-memory-server": "^8.12.2",
"mongoose": "^7.1.1",

import * as mongoose from 'mongoose';
import { MongoMemoryServer } from 'mongodb-memory-server';

let con: mongoose.Connection;
let mongoServer: MongoMemoryServer;

export const connectDB = async () => {
  mongoServer = await MongoMemoryServer.create();
  await mongoose.connect(mongoServer.getUri(), {})
  .catch((err) => {
    console.log(err);
  });
  con = mongoose.connections[0];
  console.log(con.readyState);
}

我通过如上所述将mongoose.connection切换到mongoose.connections [0]解决了这个问题。我不确定这是mongoose 7.2.0的问题还是我的工作区配置的问题。