Function

A virtual value that is not stored in the DB. This could for example be useful if you want to provide a default value in your model that is returned to the user but not stored in the DB.

You could also use it to validate a value before permuting and storing it. Checking password length before hashing it for example:

sequelize.define('user', {
  password_hash: DataTypes.STRING,
  password: {
    type: DataTypes.VIRTUAL,
    set: function (val) {
       this.setDataValue('password', val); // Remember to set the data value, otherwise it won't be validated
       this.setDataValue('password_hash', this.salt + val);
     },
     validate: {
        isLongEnough: function (val) {
          if (val.length < 7) {
            throw new Error("Please choose a longer password")
         }
      }
    }
  }
})

In the above code the password is stored plainly in the password field so it can be validated, but is never stored in the DB.

VIRTUAL also takes a return type and dependency fields as arguments If a virtual attribute is present in attributes it will automatically pull in the extra fields as well. Return type is mostly useful for setups that rely on types like GraphQL.

{
  active: {
    type: new DataTypes.VIRTUAL(DataTypes.BOOLEAN, ['createdAt']),
    get: function() {
      return this.get('createdAt') > Date.now() - (7 * 24 * 60 * 60 * 1000)
    }
  }
}