为什么“服务器已连接”出现在快速应用程序中 VScode 终端中的“数据库已连接”之前？

Question

I just want to know Why "Server is Connected" comes before the "Database is connected" in terminal of VScode in express app?

Here is my code

const express = require("express"); 
const cors = require("cors"); 
const mongoose = require("mongoose"); 
const exercises = require("./routes/exercises"); 
const users = require("./routes/users"); 

require('dotenv').config(); 
const app = express(); 


const port =  process.env.PORT || 8000; 

app.use(cors()); 
app.use(express.json()); 


mongoose.connect("mongodb+srv://@cluster0.lzvul.mongodb.net/my_database?retryWrites=true&w=majority&useNewUrlParser=true&useUnifiedTopology=true");
  const db = mongoose.connection;
  db.on("error", console.error.bind(console, "connection error: "));
  db.once("open", () => {
    console.log("Connected successfully");
  });
  
app.listen(port , () => { 
    console.log(`Server is running on localhost:${port}`); 
})

Answer 1

First, take care of your credentials, use the.env file just like you used it for storing the port.

Now, talking about what happened to your code. The mongoose.connection function returns a Promise , which means, a peace of code that will run along side the rest of your code if you don't specify that you want to wait for it to respond back. For this reason, the server starts running before the database is properly connected, it takes longer for your application to get in contact with mongodb servers than it takes for express to get running.

I strongly recommend you to have a look at the Promise documentation to understand it deeper.

Anyway a possible solution for know is to await for the mongoose connection to be stablished and just then start your server

...
/* 
Assuming you've put your mongodb URI in the MONGO_URI variable 
at the .env file
*/
mongoose.connect(process.env.MONGO_URI)
  .then(() => {
    const db = mongoose.connection;
    db.on("error", console.error.bind(console, "connection error: "));
    db.once("open", () => {
      console.log("Connected successfully");
    });
  
    app.listen(port , () => { 
      console.log(`Server is running on localhost:${port}`); 
    });
  })
  .catch((err) => {
    console.log("Something went wrong with the database connection");
  });

By using the .then method we are basically saying, we want to wait for this function to respond, and when it responds we want to execute this following function, where we call the rest of logic you wrote, server startup included.

That will make sure that the database initializes before the server and if something goes wrong with this process the server doesn't start.