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

The Query.prototype.lean() function return the documents from queries with the lean option enabled are plain JavaScript objects, not Mongoose Documents.

Syntax:

Query.prototype.lean()

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

Mongoose Installation:

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:

index.js

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.find().lean(true)
console.log("Lean Value:", query._mongooseOptions);

The project structure will look like this: 

Run index.js file using below command: 

node index.js

Output:

Lean Value: { lean: true }

Example 2:

index.js

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.find().lean(false)
console.log("Lean Value:", query._mongooseOptions);

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
Lean Value: { lean: false }

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