How does Query.prototype.Symbol.asyncIterator() works in Mongoose ?

The Query.prototype.Symbol.asyncIterator() function is used to return an asyncIterator for use with for/await/of loops. And it works for find() queries. User does not need to call this function explicitly, it gets automatically called by the JavaScript runtime.
Syntax:

Query.prototype.Symbol.asyncIterator()

Parameters: This function does not have any parameter.
Return Value: This function Returns an asyncIterator for use with for/await/of loops.

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

Database: The sample database used here is shown below.

After that, you can just create a folder and add a file for example, index.js as 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 }
});

const Query = User.find();
console.log("Value: ", Query.asyncIterator)

Run index.js file using below command: 

node index.js

Output:

Value:  undefined

Note: If Symbol.asyncIterator is undefined, that means your Node.js version does not support async iterators. 

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();
console.log("Using Express: ", Query.asyncIterator)

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:

Using Express: undefined

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