Mongoose Query.prototype.$where() API

The Mongoose Query API.prototype.$where() method of the Mongoose API is used on the Query objects. It allows us to put the where condition in the form of JavaScript object or a function in order to pass the expression to the mongodb system. Let us understand the $where() method using an example.

Syntax:

query.$where( js );

Parameters: This method accepts a single parameter as described below:

• js: It is used to specify the JavaScript expression or function.

Return Value: This method returns query object.

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: 

Database Structure: The database structure will look like this, the following database present in the MongoDB.

Example 1: The below example illustrates the basic functionality of the Mongoose Connection $where() method. We are fetching the document from collection where name is equal to “Student1”.

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: { type: String, required: true },
    age: Number,
    rollNumber: { type: Number, required: true },
});

const StudentModel = connectionObject.model('Student', studentSchema);

const query = StudentModel.find();
query.$where("this.name == 'Student1'");
query.then(res => {
    console.log(res);
}).catch(err => console.log(err));

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

node app.js

Output:

[
  {
    _id: new ObjectId("63c2fe2ef9e908eb17f225da"),
    name: 'Student1',
    age: 25,
    rollNumber: 36,
    __v: 0
  }
]

Example 2: The below example illustrates the basic functionality of the Mongoose Connection $where() method. We are fetching the documents from the collection where age is greater than and equal to 18.

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: { type: String, required: true },
    age: Number,
    rollNumber: { type: Number, required: true },
});

const StudentModel = connectionObject.model('Student', studentSchema);

const query = StudentModel.find();
query.$where(function () {
    return this.age >= 18;
});
query.exec((error, result) => {
    if (error) {
        console.log("Error -", error);
    } else {
        console.log("Result -", result);
    }
})

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

node app.js

Output:

Result - [
  {
    _id: new ObjectId("63c2fe2ef9e908eb17f225da"),
    name: 'Student1',
    age: 25,
    rollNumber: 36,
    __v: 0
  },
  {
    _id: new ObjectId("63c2fe2ef9e908eb17f225db"),
    name: 'Student2',
    age: 18,
    rollNumber: 65,
    __v: 0
  }
]

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