How does Query.prototype.$where() work in Mongoose?

Last Updated : 11 Apr, 2023

The Query.prototype.$where() specifies a JavaScript function or expression to pass to MongoDB’s query system. Using this, the where condition can be customized according to the need of the user.

Syntax:

Query.prototype.$where()

Parameters: This function has one parameter js i.e. JavaScript string or function.
Return Value: This function returns Query object.
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:

Project Structure: The project structure will look like this:

Example 1:

Filename: index.js

javascript

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();  
query.collection(User.collection); 
  
query.$where(function () { 
    if (this.age < 100) return true; 
    else return false; 
}) 
  
query.where('age').gte(1).exec(function(error, result) { 
    if (error) { 
        console.log(error) 
    } else { 
        console.log(result) 
    } 
}); 

Run index.js file using below command:

node index.js

Output:

[
  { _id: 5ebb9129a99bde77b2efb809, 
    name: 'Gourav', age: 10, __v: 0 },
  { _id: 5ebc3669a99bde77b2efb9ba, 
    name: 'Lalit', age: 25, __v: 0 }, 
  { _id: 5ebc367da99bde77b2efb9bf,
    name: 'Piyush', age: 5, __v: 0 }, 
  { _id: 5ebd345f5d2d8a3534b2f391,
    name: 'Manish', age: 34, __v: 0 } 
]

Example 2:

Filename: index.js

javascript

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();  
query.collection(User.collection); 
  
query.$where(function () { 
    if (this.age < 100) return false; 
    else return true; 
}) 
  
query.where('age').gte(1).exec(function(error, result) { 
    if (error) { 
        console.log(error) 
    } else { 
        console.log(result) 
    } 
}); 