Day 8: Introduction to Classes

Phase 2: Object-Oriented TypeScript

Welcome to Phase 2! We are moving from functional scripts to structured, object-oriented applications. The core of OOP is the Class.

What is a Class?

A class is a blueprint for creating objects. You define the properties (data) and methods (behavior) that the objects will have.

class Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hello, my name is ${this.name}.`);
  }
}

Creating Instances

To use the class, we create an instance of it using the new keyword.

const jack = new Person("Jack", 25);
jack.greet(); // "Hello, my name is Jack."

The this Keyword

Inside a class, this refers to the current instance of the object. this.name accesses the name property of that specific person.

Initialization & Strict Property Initialization

TypeScript is strict. If you declare a property like name: string, you must initialize it in the constructor.

class Car {
  model: string; // Error: Property 'model' has no initializer...
}

To fix this, either:

1. Initialize it in the constructor.
2. Give it a default value: model: string = "Toyota";.
3. Mark it as optional: model?: string;.
4. Use the Definite Assignment Assertion (not recommended for beginners): model!: string;.

Challenge for Today

1. Create a class Calculator.
2. It should have no properties.
3. Add methods add(a, b), subtract(a, b), multiply(a, b), divide(a, b).
4. Type the parameters and return values properly.
5. Create an instance and calculate add(5, 10).

See you on Day 9, where we control access with Access Modifiers!