在 MongoDB 3.6 NodeJS 驱动程序中断言？ 以及在使用promise实现时如何使用assert？

Question

In MongoDB 3.6 driver, when I connect to mongodb host by callback, I follow the quickstart guide:

From the example:

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected successfully to server");

  const db = client.db(dbName);

  client.close();
});

What is the assert function usage? And if implement in promise method, how can I use assert?

From my project, I don't know where I should put "assert" to the function. Below is my server.js

import express from 'express';
import path from 'path';
import bodyParser from 'body-parser';
import { MongoClient } from 'mongodb';
import assert from 'assert';

const app = express();
const MONGO_URL = 'mongodb://localhost:27017';
const dbName = 'inventory';
app.use(express.static('../public'));
app.use(bodyParser.json());

app.get('/api/issues', (req, res) => {
  db.collection('issues').find().toArray().then(issues => {
    const metadata = { total_count: issues.length };
    res.json({ _metadata: metadata, records: issues })
  }).catch(error => {
    console.log(error);
    res.status(500).json({ message: `Internal Server Error: ${error}` });
  });
});

let db;
MongoClient.connect(MONGO_URL).then(client => {
  db = client.db(dbName);

  app.listen(4000, () => {
    console.log('App started on port 4000');
  });
}).catch(error => {
  console.log('ERROR:', error);
});

Answer 1

When you use callback style, when an error occurs, the err argument will have such error and thus will not be null .

So that assertion:

MongoClient.connect(url, function(err, client) {
  assert.equal(null, err); // guarantees err is null, so no error has occurred

Exists to make sure err is null before the code continues.

In **Promise style**, you don't need to assert. If an error occurs, the `.catch` will be called.

So, what you needed to to was to declare a .catch() in your promise, which you already do in:

 MongoClient.connect(MONGO_URL).then(client => { ... }).catch(error => { // this catch will be called when an error occurs console.log('ERROR:', error); });

Note: Lastly, if you used async / await you had to declare a catch of a try/catch block to handle the errors. Eg

(async () => { try { let client = await MongoClient.connect(MONGO_URL) ... } catch(error) { // this catch will be called when an error occurs console.log('ERROR:', error); } })();