JavaScript and MongoDB issue -- TypeError: Unable to access properties of an undefined value (attempting to read 'collection')

Question

JavaScript and MongoDB issue -- TypeError: Unable to access properties of an undefined value (attempting to read 'collection')

Encountered an error in the terminal or command prompt as discussed below:

The specific error message is:

const taskCollection = db.collection('tasks');
                              ^

TypeError: Cannot read properties of undefined (reading 'collection')
    at Command.<anonymous> (C:\Abc\xyz\Task-Manager-CLI-Project\index.js:50:31)
    at Command.listener [as _actionHandler] (C:\Abc\xyz\Task-Manager-CLI-Project\node_modules\commander\lib\command.js:494:17)
    ...

This pertains to a task manager CLI project linked with mongoDB, yet the persistent error remains unresolved. It's unclear where the issue lies. How can this be rectified? Below are snippets from two code files:

db.js

...
module.exports = {connect, getDB};

index.js

...
program.parse(process.argv);

Attempted altering the URI in MongoDB compass and changed it from "New Connection" to "Project1", updated the URI in db.js. No success.
Switched the URI from mongodb://localhost:27017 to mongodb://localhost:27018 or modified it to mongodb://localhost:27017/Project1. Still facing the same error.

The desired outcome is for commands list, add, delete to operate correctly in the command prompt based on the coded logic. However, the system reports that the collection tasks within database task_manager_db is undefined, leading to property-read errors.

javascript mongodb typeerror uri mongodb-compass

Answer 1

Answer №1

As connect() operates asynchronously, the function should return db within itself and implement await in your index.js script as shown below:

db.js

const { MongoClient } = require('mongodb');
const uri = 'mongodb://localhost:27017';
const client = new MongoClient(uri)

let db;

async function connect(){
    try{
        await client.connect();
        db = client.db('task_manager_db'); 
        return db;
    } catch(err){
        console.log('An error occurred while connecting to the database...', err);
    }
}

module.exports = {connect};

index.js

const program = require('commander');
const {connect} = require('./db');

program
.version('1.0.0')
.description('Task Manager CLI');

program
.command('list')
.description('display a list of commands')
.action(async() => {
    const db = await connect();
    const taskCollection = db.collection('tasks');

    try{
        const tasks = await taskCollection.find().toArray();
        console.log("Listing all tasks:");
        tasks.forEach((task) => {
            console.log(`- ${task.task}`);
        });
   }catch(err){
       console.log("Error displaying tasks...", err);
   }

});

program
.command('add <taskname>')
.description('Include a new task')
.action(async(taskname) => {
    const db = await connect();
    const taskCollection = db.collection('tasks');

    try{
        const result = await taskCollection.insertOne({ task: taskname });
        console.log(`New Task added: ${taskname} TaskID: ${result.insertedId}`);
    }catch(err){
        console.log("Error adding task...", err);
    }
});

program
.command("delete <TaskId>")
.description("Remove a task")
.action(async(TaskId) => {
    const db = await connect();
    const taskCollection = await db.collection('tasks');

    try{
        const result = await taskCollection.deleteOne({ _id: new ObjectId(TaskId) });
        console.log(`Deleted Task (ID: ${TaskId})`);
    }catch(err){
        console.log("An error occurred while deleting a task...", err);
    }
});

program.parse(process.argv);

Answer 2

As connect() operates asynchronously, the function should return db within itself and implement await in your index.js script as shown below:

db.js

const { MongoClient } = require('mongodb');
const uri = 'mongodb://localhost:27017';
const client = new MongoClient(uri)

let db;

async function connect(){
    try{
        await client.connect();
        db = client.db('task_manager_db'); 
        return db;
    } catch(err){
        console.log('An error occurred while connecting to the database...', err);
    }
}

module.exports = {connect};

index.js

const program = require('commander');
const {connect} = require('./db');

program
.version('1.0.0')
.description('Task Manager CLI');

program
.command('list')
.description('display a list of commands')
.action(async() => {
    const db = await connect();
    const taskCollection = db.collection('tasks');

    try{
        const tasks = await taskCollection.find().toArray();
        console.log("Listing all tasks:");
        tasks.forEach((task) => {
            console.log(`- ${task.task}`);
        });
   }catch(err){
       console.log("Error displaying tasks...", err);
   }

});

program
.command('add <taskname>')
.description('Include a new task')
.action(async(taskname) => {
    const db = await connect();
    const taskCollection = db.collection('tasks');

    try{
        const result = await taskCollection.insertOne({ task: taskname });
        console.log(`New Task added: ${taskname} TaskID: ${result.insertedId}`);
    }catch(err){
        console.log("Error adding task...", err);
    }
});

program
.command("delete <TaskId>")
.description("Remove a task")
.action(async(TaskId) => {
    const db = await connect();
    const taskCollection = await db.collection('tasks');

    try{
        const result = await taskCollection.deleteOne({ _id: new ObjectId(TaskId) });
        console.log(`Deleted Task (ID: ${TaskId})`);
    }catch(err){
        console.log("An error occurred while deleting a task...", err);
    }
});

program.parse(process.argv);

Answer 3

Answer №2

The problem lies in the connection to MongoDB, which operates asynchronously. To ensure the connection is established before proceeding with any commands, use the async function in your commander action. When dealing with index.js, the snippet for connecting to Mongo should resemble the following:

program
.version('1.0.0')
.description('Task Manager CLI')
//connecting MongoDB database 
.action(async ()=>{
   await connect());
}

This approach guarantees that your connection is fully established prior to executing any actions.

Insights on async actions in commander.js

Understanding why MongoDB shows "Cannot read properties of undefined (reading 'collection')" error

Answer 4

The problem lies in the connection to MongoDB, which operates asynchronously. To ensure the connection is established before proceeding with any commands, use the async function in your commander action. When dealing with index.js, the snippet for connecting to Mongo should resemble the following:

program
.version('1.0.0')
.description('Task Manager CLI')
//connecting MongoDB database 
.action(async ()=>{
   await connect());
}

This approach guarantees that your connection is fully established prior to executing any actions.

Insights on async actions in commander.js

Understanding why MongoDB shows "Cannot read properties of undefined (reading 'collection')" error

JavaScript and MongoDB issue -- TypeError: Unable to access properties of an undefined value (attempting to read 'collection')

Answer №1

Answer №2

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