How does Query.prototype.j() work in Mongoose ?

The Query.prototype.j() function is used to request acknowledgement that this operation has been persisted to MongoDB’s on-disk journal.

Syntax:

Query.prototype.j()

Parameters: This function has one val parameter of boolean type. 
Return Value: This function returns Query Object.
Installing mongoose : 

npm install mongoose

After installing the mongoose module, you can check your mongoose version in command prompt using the command. 

npm mongoose --version

After that, you can just create a folder and add a file for example, index.js as shown below.

Database: The sample database used here is shown below: 

Example 1:
Filename: index.js

javascript

const mongoose = require('mongoose');

// Database connection
mongoose.connect('mongodb://127.0.0.1:27017/geeksforgeeks', {
    useNewUrlParser: true,
    useCreateIndex: true,
    useUnifiedTopology: true
});

// User model
const User = mongoose.model('User', {
    name: { type: String },
    age: { type: Number }
});

const query = User.updateOne({name:"Gourav"}, { $set: {name: 'Geek'} }).j(true);
console.log("Above operation has been persisted to MongoDB's on-disk journal")

The project structure will look like this: 

Run index.js file using below command: 

node index.js

Output:

Above operation has been persisted to MongoDB's on-disk journal

Example 2:
Filename: index.js

javascript

const express = require('express');
const mongoose = require('mongoose');
const app = express()

// Database connection
mongoose.connect('mongodb://127.0.0.1:27017/geeksforgeeks', {
    useNewUrlParser: true,
    useCreateIndex: true,
    useUnifiedTopology: true
});

// User model
const User = mongoose.model('User', {
    name: { type: String },
    age: { type: Number }
});

const query = User.updateOne({name:"Gourav"}, { $set: {name: 'Geek'} }).j(false);
console.log("Above operation has not been persisted to MongoDB's on-disk journal")

app.listen(3000, function(error ) {
    if(error) console.log(error)
    console.log("Server listening on PORT 3000")
});

The project structure will look like this: 

Run index.js file using below command: 

node index.js

Output:

Server listening on PORT 3000
Above operation has not been persisted to MongoDB's on-disk journal

Reference:
https://mongoosejs.com/docs/api/query.html#query_Query-j