Mongoose Import for TypeScript Doesn't Work

Since you didn't share your package.json or tsconfig, it was not possible to say where the error might be. So I created new project for the code you have shared such that the error does not occur. Compare the files that I am sharing with the ones you have to narrow down your problem.

The package.json

{
  "name": "mong_type",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@types/express": "^4.11.1",
    "@types/mongoose": "^5.0.3",
    "typescript": "^2.7.2"
  },
  "dependencies": {
    "express": "^4.16.2",
    "mongoose": "^5.0.7"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "es2015",
    "module": "commonjs",
    "outDir": "./dist",
    "strict": true,
    "allowSyntheticDefaultImports": true,
    "esModuleInterop": true
  },
  "include": ["src"]
}

src/app.ts

import express from "express";
import mongoose from "mongoose";

class App {
  public express: express.Application;

  constructor() {
    this.express = express();
    this.setupDb();
  }

  private setupDb(): void {
    var mongoDb = "mongodb://127.0.0.1/my_database";
    mongoose.connect(mongoDb);
    var db = mongoose.connection;
    db.on("error", console.error.bind(console, "MongoDB Connection error"));
  }
}

This:

import mongoose from 'mongoose'

worked for me after running:

npm install mongoose @types/mongoose --save

Way more detailed explanation on why this works, here.