Stop 04 · JavaScriptTopics · 19/36

Classes

In short

When you need many objects of the same shape, like 200 students or 50 products, a class is the blueprint that stamps them out with the same fields and methods.

Syntactic sugar over prototypal inheritance that gives JavaScript a familiar, class-based way to define constructors, methods and inheritance. Classes are useful for modeling objects with shared behavior, but understanding prototypes helps explain inheritance, method lookup, and JavaScript's object model.

The jaadu mascot holds a blueprint in front of three identical, differently coloured buildings.

Think of it like this

The builder's flat plan

A housing society has one 2BHK plan: same rooms, same balcony. Every flat built from it has its own owner and its own paint colour. The class is the plan. Each object you make with new is one flat.

Watch it happen

new Topper("Meera", 1)

Your code

Topper

Student

new Topper("Meera", 1)

1/6new creates an empty object and runs Topper's constructor.

The big ideas

What you need to know

constructor sets things up

It runs when you call new Student(...) and sets the starting values on this.

Methods are shared

Methods written in the class live in one place and are shared by every object, so they don't use extra memory per object.

extends and super

class Topper extends Student reuses everything from Student. Call super() in the constructor before using this.

#private and static

#fees can only be read inside the class. static belongs to the class itself, not to each object.

See it in code

Walk through the code

1class Student {2  #fees = 0;                       // private field3  static count = 0;4  constructor(name) {5    this.name = name;6    Student.count++;7  }8  pay(amount) { this.#fees += amount; return this.#fees; }9}10 11class Topper extends Student {12  constructor(name, rank) { super(name); this.rank = rank; }13}14 15const t = new Topper("Meera", 1);16console.log(t.pay(500), Student.count);   // 500 1

1/5A private field and a static counter shared by the class.

Topper gets pay() from Student for free. Try t.#fees from outside the class and you'll get a SyntaxError, which is exactly the point.

Save yourself the pain

Common mistakes

Oops

Calling Student("Meera") without new.

Do this instead

Classes throw "Class constructor Student cannot be invoked without 'new'". Always use new Student(...).

Oops

Using this in a child constructor before calling super().

Do this instead

That throws a ReferenceError. Call super(...) first, then set your own fields.

Your mission

Model a library

~25 min

Quick check

What does this log? class A {} console.log(typeof A);

Go deeper

1Write a class with a constructor, a method and a `static` method. Validate your understanding by applying it in a small practical example.
2Practice `extends` and `super()` for inheritance between two classes. Test it with at least one normal case and one edge case so you can explain the behavior confidently.
3Learn that under the hood, classes still use prototypes.

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