# Week 4

Over the past three weeks you've transformed data with array methods, managed async flows with callbacks and Promises, and fetched from a real API. But all your data has been plain objects - no validation, no built-in behavior, no structure beyond what you remember to add each time.

This week you'll learn classes: blueprints for creating objects with consistent structure and built-in behavior. You'll build Tea, Order, and Inventory classes that validate data, calculate totals, and work together - just like the models behind the Tea Shop API you've been using.

## Contents

* [Preparation](/course-content/backend/advanced-javascript/week4/preparation.md)
* [Slides](https://github.com/HackYourFuture-CPH/program/blob/main/courses/backend/advanced-javascript/week4/session-materials/slides/index.html)
* [Session Plan](/course-content/backend/advanced-javascript/week4/session-plan.md) (for mentors)
* [Session Materials](https://github.com/HackYourFuture-CPH/program/blob/main/courses/backend/advanced-javascript/week4/session-materials/README.md)
* [Assignment](/course-content/backend/advanced-javascript/week4/assignment.md)

## Learning Goals

By the end of this session, you will be able to:

* [ ] Understand why classes exist (organizing data with behavior)
* [ ] Declare a class using `class`, `constructor`, and `this`
* [ ] Instantiate objects from classes using `new`
* [ ] Add methods to classes and call them on instances
* [ ] Use `this` to access and modify instance state
* [ ] Use static methods for utility operations on a class
* [ ] Use inheritance with `extends` and `super()`
* [ ] Understand the difference between a class (blueprint) and an object (instance)

### After this session you will understand code like this

```js
// From scattered plain objects...
const tea = { name: "Sencha", pricePerGram: 0.12, origin: "Japan" };
// No validation, no behavior, no consistency

// ...to structured classes with built-in behavior
class Tea {
  constructor(name, pricePerGram, origin) {
    this.name = name;
    this.pricePerGram = pricePerGram;
    this.origin = origin;
  }

  priceFor(grams) {
    return this.pricePerGram * grams;
  }
}

const sencha = new Tea("Sencha", 0.12, "Japan");
console.log(sencha.priceFor(100)); // 12
```


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