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

The Query.prototype.cast() function casts this query to the schema of the model. And if the obj is present, then it is casted instead of this query.
Syntax:

Query.prototype.cast()

Parameters: This function has one parameter i.e. the model to cast and if not set, then the default is this.model.
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.

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()
var res = query.cast();

console.log("Casted to self:", res)

The project structure will look like this:

Run index.js file using below command: 

node index.js

Output:

Casted to self: {}

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()
var res = query.cast();

console.log("Self Casting:", res)

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
Self Casting: {}

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