TypeError: Cannot read properties of undefined (reading 'db')

Question

I am trying to use the following class db.js to operate mongodb

'use strict'
const { MongoClient } = require('mongodb');
const API = {
    db: {
        db_url: 'mongodb://127.0.0.1:27017',
        db_test: {
            db_name: 'test'
        }
    }
};
class Db {
    dbClient = null;
    dbUrl;
    dbName;
    static getInstance(dbUrl, dbName) {
        if (!Db.instance) {
            Db.instance = new Db(dbUrl, dbName);
        }
        return Db.instance;
    }

    constructor(dbUrl, dbName) {
        this.dbUrl = dbUrl;
        this.dbName = dbName;
        this.connect();
    }

    connect() {
        return new Promise((resolve, reject) => {
            if (this.dbClient) {
                resolve(this.dbClient);
            }
            MongoClient.connect(this.dbUrl, {useUnifiedTopology: true}, (err, connectedClient) => {
                if (err) {
                    reject(err);
                }
                this.dbClient = connectedClient.db(this.dbName);
                resolve(this.dbClient);
            });
        });
    }

    find(tablename, json) {
        return new Promise((resolve, reject) => {
            this.connect()
                .then(db => {
                    const result = db.collection(tablename).find(json);
                    result.toArray((err, data) => {
                        if (err) {
                            reject(err);
                        }
                        resolve(data);
                    });
                })
                .catch(err => {
                    reject(err);
                });
        });
    }
}

module.exports = Db.getInstance(API.db.db_url, API.db.db_test.db_name);

But I always get an error when it runs, it gives

/home/admin/depis/node_modules/mongodb/lib/utils.js:698
          throw error;
          ^

TypeError: Cannot read properties of undefined (reading 'db')
    at /home/admin/depis/src/lib/db.js:41:49
    at /home/admin/depis/node_modules/mongodb/lib/utils.js:695:9
    at /home/admin/depis/node_modules/mongodb/lib/mongo_client.js:285:23
    at connectCallback (/home/admin/depis/node_modules/mongodb/lib/operations/connect.js:367:5)
    at /home/admin/depis/node_modules/mongodb/lib/operations/connect.js:554:14
    at Object.connectHandler [as callback] (/home/admin/depis/node_modules/mongodb/lib/core/sdam/topology.js:286:11)
    at Timeout._onTimeout (/home/admin/depis/node_modules/mongodb/lib/core/sdam/topology.js:443:25)
    at listOnTimeout (node:internal/timers:559:17)
    at processTimers (node:internal/timers:502:7)

The line 41 in db.js is

this.dbClient = connectedClient.db(this.dbName);

But there seems to be no error in the code here, please help.

Make sure you stop processing if an error occurs. Try `return reject(err);`. You should do this every time you use `reject()` or at least put the following code into an `else` block — Phil, Jul 21 '22 at 00:56
Does this answer your question? [Is it possible to stop JavaScript execution?](https://stackoverflow.com/questions/9298839/is-it-possible-to-stop-javascript-execution) — Phil, Jul 21 '22 at 00:57
@Phil Could you please give me an example? I use return for reject() and resolve() in above code, thank you. — Bill, Jul 21 '22 at 01:15
I'm not sure what else to add... just make sure your code doesn't continue executing when it should not. Add `return` before each `resolve()` and `reject()` call — Phil, Jul 21 '22 at 01:16

TypeError: Cannot read properties of undefined (reading 'db')

0 Answers0