Class method VS function in an object property

JavaScriptJunior~2 min read

1. Syntax

Class method:

javascript

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

Object property function:

javascript

const user = {
  greet: function() {
    console.log(`Hi, ${this.name}`);
  }
};

They look similar, but the first one is a prototype method, while the second is a unique property of that specific object.

2. Where the Method Is Stored

Approach	Where the function is stored
Class method	In `User.prototype`
Object property function	In the object itself (in its "body")

Example:

javascript

class A {
  sayHi() {}
}

const a1 = new A();
const a2 = new A();

console.log(a1.sayHi === a2.sayHi); // true: one method shared by everyone

// Now the object
const obj1 = {
  sayHi: function() {}
};
const obj2 = {
  sayHi: function() {}
};

console.log(obj1.sayHi === obj2.sayHi); // false: each one has its own copy

Class methods save memory, because a single method is stored on the prototype and used by all instances.

3. Creation and When the Function Is Allocated

In a class, the method is added once, when the class is defined.
In an object, the property function is created every time a new object is created.

So if you create 1000 instances of a class, they all share one method. But if you create 1000 objects with a property function, there will be 1000 separate functions in memory.

4. Difference With Inheritance

Class methods automatically participate in inheritance via prototype.

javascript

class Animal {
  speak() {
    console.log("The animal makes a sound");
  }
}

class Dog extends Animal {}

const dog = new Dog();
dog.speak(); // The animal makes a sound

An object with a property function, on the other hand, does not participate in inheritance (unless you use Object.create).

5. Difference in `super` Syntax

super (calling the parent's method) works only in classes, and not in plain objects (unless you use advanced tricks with Object.setPrototypeOf).

javascript

class A {
  greet() { console.log("Hi from A"); }
}
class B extends A {
  greet() {
    super.greet(); // available
    console.log("Hi from B");
  }
}

In an object:

javascript

const a = { greet() { console.log("Hi from A"); } };
const b = {
  greet() {
    // super.greet(); doesn't work without a prototype
    console.log("Hi from B");
  }
};

6. `this` Behavior

Both in a class and in an object, the method keeps its this, but if you pass it as a callback, this is lost (unless it's an arrow function):

javascript

class A {
  hello() {
    console.log(this); // A
  }
}

const a = new A();
setTimeout(a.hello, 1000); // undefined (context lost)

The same happens with an object:

javascript

const obj = {
  hello() {
    console.log(this);
  }
};

setTimeout(obj.hello, 1000); // undefined

If you need to preserve the context, arrow functions are used as properties:

javascript

class A {
  hello = () => {
    console.log(this); // always the correct this
  };
}

7. Difference at the Language Level

Difference	Class Method	Object Property Function
Where it's stored	`Class.prototype`	In the object itself
Shared across instances	Yes	No
Created when	The class is defined	The object is created
Participates in inheritance	Yes	No
Access to `super`	Yes	No
Loses `this` in a callback	Yes (unless an arrow function)	Yes (unless an arrow function)
Performance and memory	More efficient	Less efficient

Example for Visualization

javascript

class User {
  greet() { console.log("I am a class method"); }
}

const obj = {
  greet() { console.log("I am an object property function"); }
};

console.log(Object.getOwnPropertyNames(User.prototype)); // ["constructor", "greet"]
console.log(Object.keys(obj)); // ["greet"]

Summary: Class methods are functions on the prototype, shared by all instances (they save memory, participate in inheritance, and support super). Object property functions are regular properties that belong to a specific object, are recreated every time, and do not participate in inheritance.

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