How to add custom function to sequelize.js in Node.js ?

When using Sequelize ORM in Node.js, you might encounter situations where you need to add custom functions to address specific business logic. Below, I'll outline the steps to add custom methods to Sequelize models and provide a concrete example to illustrate the process.

Step 1: Create the Model

First, ensure you have a Sequelize model. Suppose we have a model named User where we want to add a custom function to check if a user's age meets a specific value.

javascript
const { Model, DataTypes } = require('sequelize');
const sequelize = new Sequelize('sqlite::memory:');

class User extends Model {}

User.init({
  username: DataTypes.STRING,
  birthday: DataTypes.DATE
}, { sequelize, modelName: 'user' });

sequelize.sync();

Step 2: Add Instance Methods or Class Methods

In Sequelize, you can add instance methods or class methods:

Instance Methods

Instance methods are functions defined on model instances. These methods can operate on instance data.

javascript
User.prototype.isOldEnough = function (age) {
  const now = new Date();
  const currentAge = now.getFullYear() - this.birthday.getFullYear();
  return currentAge >= age;
};

In this example, the isOldEnough method checks if the user has reached the specified age.

Class Methods

Class methods are defined on the model class. They do not depend on specific instances.

javascript
User.isAdult = function () {
  return this.findAll({
    where: {
      birthday: {
        [Sequelize.Op.lte]: new Date(new Date() - 18 * 365 * 24 * 60 * 60 * 1000) // 18 years ago
      }
    }
  });
};

Here, isAdult is a class method used to find all users who are at least 18 years old.

Step 3: Use Custom Functions

After creating custom methods, you can call them in other parts of your application.

javascript
(async () => {
  const newUser = await User.create({
    username: 'johndoe',
    birthday: new Date(2000, 0, 1)
  });

  console.log(newUser.isOldEnough(18)); // Outputs: true or false

  const adults = await User.isAdult();
  console.log(adults); // Outputs all adult users
})();

Summary

By adding instance methods and class methods to the model, you can enhance Sequelize models with powerful functionality, enabling you to implement complex business logic in a highly flexible manner. This approach not only makes the code more modular but also improves maintainability and readability. In the example above, we demonstrate how to determine if a user meets a specific age requirement based on their birthday, which is a common need in many applications.