Validation and Side Effects

Part of the Object Oriented Programming section of Coddy's JavaScript journey. Lesson 45 of 56.

Getters and setters can do more than just read and write properties - they can validate inputs and trigger side effects when values change.

Let's create a User class with validation:

class User {
  constructor(name, age) {
    this._name = name;
    this._age = age;
  }
  
  get age() {
    return this._age;
  }
  
  set age(value) {
    // Validate age is a number and within reasonable range
    if (typeof value !== 'number') {
      throw new Error('Age must be a number');
    }
    
    if (value < 0 || value > 120) {
      throw new Error('Age must be between 0 and 120');
    }
    
    this._age = value;
    
    // Side effect: log when age changes
    console.log(`Age updated to ${value}`);
  }
}

Now when we use the setter, validation happens automatically:

const user = new User('Alice', 25);

// This works
user.age = 30;  // Logs: "Age updated to 30"

// This throws an error
user.age = -5;  // Error: Age must be between 0 and 120

// This also throws an error
user.age = "thirty";  // Error: Age must be a number

Validation = Checking if a value is valid before storing it
Side Effects = Additional actions that happen when a property changes

Challenge

You're given a BankAccount class with a basic setter. Your task is to complete the balance setter.

Add to the setter:

Validation: Check if value is not a number OR is less than 0 - if so, console.log("Invalid balance") and use return to exit the setter
Side Effect: After storing a valid value, console.log(\`Balance updated to $${value}\`)

Try it yourself

import { BankAccount } from './account.js';

// Test cases
const account = new BankAccount("Savings");

// Test 1: Valid balance
account.balance = 100;
console.log(account.balance);

// Test 2: Invalid balance (negative)
account.balance = -50;
console.log(account.balance);

// Test 3: Invalid balance (not a number)
account.balance = "abc";
console.log(account.balance);