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

The Query.prototype.catch() function executes the query and returns a promise which can be resolved with either the doc(s) or rejected with the error.
Syntax:

Query.prototype.catch()

Parameters: This function has one parameter i.e. a reject callback function.
Return Value: This function returns a promise.

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:

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 promise = User.find({age:"Not a number"});

promise.catch(function(error) {
    console.log("Error Occurred:", error.message)
})

The project structure will look like this: 

Run index.js file using below command: 

node index.js

Output:

Error Occurred: Cast to Number failed for value 
"Not a number" at path "age" for model "User"

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 promise = User.find({ age: "Not a number" });

promise.catch(function(error) {
    console.log("Error Occurred:", error.message)
})

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
Error Occurred: Cast to Number failed for value 
"Not a number" at path "age" for model "User"

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