Mongoose Query.prototype.exists() API

The Mongoose Query API exists() method is used to find the documents that contain the fields ([path]) if the “val” parameter is true, otherwise it returns the documents that do not contain the field.

Syntax:

Query.prototype.exists([path], val)

Parameters: It accepts the following parameters as mentioned above and described below:

• path: It is a string that identifies the field that is present (or not present) in the documents.
• val: It is a boolean that determines whether to find or not find the fields in the documents.

Return type: It returns a Query object as a response.

Creating node application And Installing Mongoose:

Step 1: Create a node application using the following command:

mkdir folder_name
cd folder_name
npm init -y
touch main.js

Step 2: After completing the Node.js application, Install the required module using the following command

npm install mongoose

Example 1: In this example, we will use this method to find the documents that contain the field “name”.

Filename: main.js

Javascript

const mongoose = require('mongoose')

// Database connection
mongoose.connect('mongodb://localhost:27017/query-helpers',
    {
        dbName: 'event_db',
        useNewUrlParser: true,
        useUnifiedTopology: true
    }, err => err ? console.log(err)
        : console.log('Connected to database'));

const personSchema = new mongoose.Schema({
    name: {
        type: String,
    },
    age: {
        type: Number,
    }
});

const personsArray = [
    {
        name: 'Luffy',
    },
    {
        name: 'Nami',
        age: 20,
    },
    {
        age: 35
    }
]

const Person = mongoose.model('Person', personSchema);

(async () => {
    await Person.insertMany(personsArray);
    const res = await Person.where('name').exists();
    console.log({ res });
})()

Step to Run Application: Run the application using the following command from the root directory of the project:

node main.js

Output: 

GUI Representation of the  Database using MongoDB Compass:

Example 2:  In this example, we will use this method to find the documents that contain the field “age”.

Filename: main.js

Javascript

const mongoose = require('mongoose')

// Connecting to the database
mongoose.connect('mongodb://localhost:27017/query-helpers',
    {
        dbName: 'event_db',
        useNewUrlParser: true,
        useUnifiedTopology: true
    }, err => err ? console.log(err)
        : console.log('Connected to database'));

const personSchema = new mongoose.Schema({
    name: {
        type: String,
    },
    age: {
        type: Number,
    }
});

const personsArray = [
    {
        name: 'Luffy',
    },
    {
        name: 'Nami',
        age: 20,
    },
    {
        age: 35
    }
]

const Person = mongoose.model('Person', personSchema);

(async () => {
    await Person.insertMany(personsArray);
    const res = await Person.where('age').exists();
    console.log({ res });
})()

Step to Run Application: Run the application using the following command from the root directory of the project

node main.js

Output: 

GUI Representation of the  Database using MongoDB Compass:

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