JavaScript Prototype Chain & Prototypal Inheritance Interview Guide

Unlike classical object-oriented languages (like Java or C++), JavaScript uses Prototypal Inheritance. Every object in JavaScript has an internal link to another object called its prototype.

How Prototype Property Lookup Works

When accessing a property on an object: 1. JavaScript checks if the property exists directly on the object (own property). 2. If not found, it traverses up the [[Prototype]] link to the parent prototype object. 3. This traversal continues until the property is found or null is reached (end of the chain).

const parent = {
  greet() {
    return `Hello from ${this.name}`;
  }
};

const child = Object.create(parent);
child.name = "Alice";

console.log(child.greet()); // "Hello from Alice" (inherited method)
console.log(child.hasOwnProperty("name")); // true
console.log(child.hasOwnProperty("greet")); // false

ES6 class vs Prototypal Constructor Functions

ES6 class syntax is syntactic sugar built on top of prototypal inheritance.

// ES6 Class
class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    return `${this.name} makes a sound.`;
  }
}

// Equivalent ES5 Constructor Function
function AnimalES5(name) {
  this.name = name;
}
AnimalES5.prototype.speak = function () {
  return this.name + " makes a sound.";
};