How does prototypal inheritance work?

JavaScript does not copy members from a parent class into a child. Each object has an internal link to another object, its prototype. Property access follows that link at lookup time, so inherited behaviour stays live.

const animal = {
  eats: true,
  walk() {
    console.log("walking");
  },
};

const rabbit = Object.create(animal);
rabbit.jumps = true;

console.log(rabbit.eats); // true, inherited
console.log(Object.hasOwn(rabbit, "eats")); // false
rabbit.walk(); // "walking"

Prototype vs __proto__

• Object.getPrototypeOf(obj) reads the prototype safely
• Object.setPrototypeOf(obj, proto) changes it, but is slow and discouraged in hot code
• obj.__proto__ is a legacy accessor for the same link
• Fn.prototype is the object that instances created with new Fn() will use as their prototype

Classes are syntax over the same mechanism. extends sets up the prototype chain, and super looks up the parent prototype.

class Animal {
  speak() {
    return "sound";
  }
}

class Dog extends Animal {
  speak() {
    return super.speak() + ": woof";
  }
}

console.log(new Dog().speak()); // "sound: woof"

Why it matters

• Shared methods live once on the prototype instead of on every instance
• instanceof walks the prototype chain
• Changing a prototype method affects existing instances
• Mutating built-in prototypes such as Array.prototype is risky and usually avoided