What does this do inside a class?

JavaScriptJunior~2 min read

1. What Is `this` in a Class

this inside a class is a reference to the current instance (object) created with new.

Example:

javascript

class User {
  constructor(name) {
    this.name = name; // this points to the object being created
  }

  greet() {
    console.log(`Hi, ${this.name}!`);
  }
}

const u = new User("Alice");
u.greet(); // Hi, Alice!

Here:

At the moment new User("Alice") runs, a new object is created.
this inside the constructor points to exactly that new object.
this.name = name writes a property onto the instance.
Then u.greet() calls the method, where this === u.

2. `this` in Class Methods

Class methods have no this of their own, it is determined at call time.

javascript

class User {
  constructor(name) {
    this.name = name;
  }

  sayHi() {
    console.log(this.name);
  }
}

const user = new User("Alice");
user.sayHi(); // this → user → "Alice"

If you call the method on its own (losing the context), this will be undefined (in strict mode):

javascript

const say = user.sayHi;
say(); // TypeError: Cannot read properties of undefined

Classes always run in strict mode, even if you never explicitly enabled it.

3. `this` in the Constructor

The constructor is called automatically with new, and this points to the new instance:

javascript

class Product {
  constructor(name, price) {
    this.name = name;
    this.price = price;
  }
}

const p = new Product("Shirt", 3000);
console.log(p.name); // Shirt

If it's not called via new → you get an error:

javascript

Product("Shirt", 3000); // TypeError: Class constructor cannot be invoked without 'new'

4. `this` With Inheritance (extends)

If a class inherits from another, you must call super() before accessing this, otherwise you get an error.

javascript

class Animal {
  constructor(name) {
    this.name = name;
  }
}

class Dog extends Animal {
  constructor(name, breed) {
    super(name); // creates this via the parent's constructor
    this.breed = breed; // this can now be used
  }
}

If super() is not called:

javascript

class Dog extends Animal {
  constructor(name) {
    this.name = name; // ReferenceError: Must call super constructor before accessing 'this'
  }
}

5. Losing the `this` Context

When a method is passed as a callback, this is lost:

javascript

class User {
  constructor(name) {
    this.name = name;
  }

  greet() {
    console.log(`Hi, ${this.name}`);
  }
}

const u = new User("Alice");
setTimeout(u.greet, 1000); // undefined, because the context is lost

Solution 1: Bind `this` Manually

javascript

setTimeout(u.greet.bind(u), 1000); // Hi, Alice

Solution 2: Use an Arrow Function

javascript

class User {
  name = "Alice";

  greet = () => {
    console.log(`Hi, ${this.name}`);
  };
}

const u = new User();
setTimeout(u.greet, 1000); // Hi, Alice

Arrow functions preserve this from the surrounding context, which is why they are often used for event handlers inside a class.

6. In Static Methods, `this` Refers to the Class Itself

javascript

class MathUtils {
  static describe() {
    console.log(this); // class MathUtils { ... }
  }
}

MathUtils.describe(); // this → MathUtils

If you extend the class:

javascript

class AdvancedMath extends MathUtils {}

AdvancedMath.describe(); // this → AdvancedMath

In other words, this in a static method refers not to an instance, but to the class itself.

7. In Private and Arrow Methods

javascript

class Counter {
  #count = 0;

  increment = () => {
    this.#count++;
    console.log(this.#count);
  };
}

const c = new Counter();
setTimeout(c.increment, 1000); // this is preserved, works correctly

Here this is not lost, because an arrow function does not create its own this.

Summary

Context	Value of `this`
In the constructor	New class instance
In a regular method	The object the method was called on
When context is lost (`setTimeout(obj.method)`)	`undefined`
In an arrow method	Inherited from the surrounding context (not lost)
In a static method	The class itself (not an instance)
Before calling `super()` in a subclass	Error: `this` is not accessible

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