Prototypes and the Prototype Chain

8 min read

How Prototypes Work

Every JavaScript object has an internal [[Prototype]] link to another object (or null). When you access a property that does not exist on an object, JavaScript walks up the prototype chain looking for it.

javascript

const animal = {
  breathe() { console.log("breathing"); }
};

const dog = Object.create(animal);
dog.bark = function() { console.log("woof"); };

dog.bark();    // woof (own property)
dog.breathe(); // breathing (found on prototype)

console.log(Object.getPrototypeOf(dog) === animal); // true

The class keyword creates the same prototype chain under the hood. Methods defined in a class body go on ClassName.prototype, not on each instance — that is what makes them memory-efficient.

javascript

class Foo {
  bar() {}
}

const f1 = new Foo();
const f2 = new Foo();
console.log(f1.bar === f2.bar); // true — same function reference

Use hasOwnProperty (or the modern Object.hasOwn(obj, key)) to distinguish own properties from inherited ones. This matters in for...in loops.

Ready to test yourself?

Practice JavaScript— quiz & coding exercises