Mongoose Schema.prototype.virtual() API

The Mongoose Schema API .prototype.virtual() method of the Mongoose API is used on the Schema object. It allows us to define virtual method and type over each schema with a specific name and method definition. Let us understand virtual() method using an example.

Syntax:

schemaObject.virtual( <name>, <options> );

Parameters: This method accepts two parameters as described below:

• name: It is used to specify the virtual method name over the schema
• options: It is used to specify various properties for that virtual method.

Return Value: This method can return any virtual type value.

Setting up Node.js Mongoose Module:

Step 1: Create a Node.js application using the following command:

npm init

Step 2: After creating the NodeJS application, Install the required module using the following command:

npm install mongoose

Project Structure: The project structure will look like this: 

Example 1: The below example illustrates the functionality of the Mongoose Schema virtual() method. In this example, we have defined a virtual named example1 on studentSchema which is return a string.

Filename: app.js

Javascript

// Require mongoose module
const mongoose = require("mongoose");

// Set Up the Database connection
const URI = "mongodb://localhost:27017/geeksforgeeks"

const connectionObject = mongoose.createConnection(URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
});

const studentSchema = new mongoose.Schema({
    name: String,
    rollNumber: Number
});

studentSchema.virtual('example1').get(() => {
    return 'Virtual Method'
});

console.log(studentSchema.virtuals.example1.getters[0].call());

Step to run the program: To run the application execute the below command from the root directory of the project:

node app.js

Output:

Virtual Method

Example 2: The below example illustrates the functionality of the Mongoose Schema virtual() method. In this example, we have defined a virtual named portNumber which is returning port number.

Filename: app.js

Javascript

// Require mongoose module
const mongoose = require("mongoose");

// Set Up the Database connection
const URI = "mongodb://localhost:27017/geeksforgeeks"

const connectionObject = mongoose.createConnection(URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
});

const studentSchema = new mongoose.Schema({
    name: String,
    rollNumber: Number
});

studentSchema.virtual('portNumber').get(() => {
    return 27017
});

console.log(studentSchema.virtuals.portNumber);

Step to run the program: To run the application execute the below command from the root directory of the project:

node app.js

Output:

VirtualType {
  path: 'portNumber',
  getters: [ [Function (anonymous)] ],
  setters: [],
  options: {}
}

Reference: https://mongoosejs.com/docs/api/schema.html#schema_Schema-virtual