Classes and Prototypes

Key Points

1. Classes are templates for creating objects.
2. When a class is instantiated:
   • Properties defined in the constructor are specific to each instance.
   • Methods are defined on the prototype.
3. JavaScript resolves method calls by:
   • First checking if the method exists on the instance.
   • If not found, it checks the prototype.

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Example Code

/**
 * Example class definition: Dog
 */
class Dog {
  name: string;
  wagTail: () => string;

  constructor(name: string) {
    this.name = name;
    this.wagTail = () => "Wagging Tail";
  }

  bark() {
    return "Barking";
  }
}

const dog1 = new Dog("Tommy");
const dog2 = new Dog("Spike");

console.log(dog1.wagTail() === dog2.wagTail()); // false
console.log(dog1.bark === dog2.bark); // true
console.log(dog1.bark === Dog.prototype.bark); // true
console.log(dog1.wagTail === Dog.prototype.wagTail); // false
console.log(dog1.wagTail === dog2.wagTail); // false

export { Dog };

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Explanation

• wagTail Property:

  • It is defined as a function inside the constructor, so each instance has its own unique version of wagTail.
  • This is why dog1.wagTail === dog2.wagTail evaluates to false.

• bark Method:

  • It is defined as part of the prototype, shared across all instances of the class.
  • This is why dog1.bark === dog2.bark and dog1.bark === Dog.prototype.bark both evaluate to true.

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