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

The Query.prototype.exec() function is used to execute the query. It can handle promises and executes the query easily. The callback can be passed as an optional parameter to handle errors and results.

Syntax:

Query.prototype.exec()

Parameters: This function has two optional parameters, one is callback function and another is operation of string or function 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

Now, create a folder and add a file, for example, index.js as shown below.

Database: The sample database used here is shown below.

Project Structure: The project structure will look like this.

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 }
});

var query = User.find();
query.exec(function(err, res){
    if(err) console.log(err)
    console.log(res)
})

Run index.js file using below command:

node index.js

Output:

[
  { _id: 5ebb9129a99bde77b2efb809, name: 'Gourav', age: 10, __v: 0 },
  { _id: 5ebc3669a99bde77b2efb9ba, name: 'Lalit', age: 25, __v: 0 },
  { _id: 5ebc367da99bde77b2efb9bf, name: 'Piyush', age: 5, __v: 0 },
  { _id: 5ebd345f5d2d8a3534b2f391, name: 'Manish', age: 34, __v: 0 }
]

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 }
});

var query = User.find({age:{$gte:20}});
query.exec(function(err, res){
    if(err) console.log(err)
    console.log(res)
})

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

Run index.js file using below command: 

node index.js

Output:

Server listening on PORT 3000
[
  { _id: 5ebc3669a99bde77b2efb9ba, name: 'Lalit', age: 25, __v: 0 },
  { _id: 5ebd345f5d2d8a3534b2f391, name: 'Manish', age: 34, __v: 0 }
]

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