Mongoose SchemaType.prototype.transform() API

Last Updated : 26 Apr, 2025

Mongoose is a MongoDB object modeling and handling for a node.js environment. Mongoose SchemaType transform is a SchemaType method that allows us to transform or manipulate a path when converting a document into JSON. Mongoose uses the current value of the path as a parameter for the method. Let’s understand more about this with some examples.

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 creating the ReactJS application, Install the required module using the following command:

npm install mongoose

Project Structure: It will look like the following.

Example 1: In this example, we will create a transform function that will give us the year of the data stored in a path.

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({ 
    date: { 
        type: Date, 
        transform: v => v.getFullYear() 
    } 
}); 
  
const Person = mongoose.model('Person', personSchema); 
const person = new Person({ date: new Date() }); 
console.log(person.toJSON().date);

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

node main.js

Output:

Example 2: In this example, we will create a transform function that will give us the full name of a person from an Array of first names and Last names.

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: Array, 
        transform: v => v.join(' ') 
    } 
}); 
  
const Person = mongoose.model('Person', personSchema); 
const person = new Person({ name: ['John', 'Doe'] }); 
console.log(person.toJSON().name);