JavaScript Prototypes: How Inheritance Really Works

JavaScript doesn't have classical inheritance — it has prototypal inheritance. Understanding the prototype chain is essential for interviews and debugging.

The Prototype Chain

Every object has an internal [[Prototype]] link. When you access a property, JS walks up the chain until it finds it or reaches null.

Constructor Functions

ES6 Classes (Syntactic Sugar)

Common Gotchas

• Object.create(null) creates an object with NO prototype — no toString, no hasOwnProperty. Used for pure dictionaries.
• hasOwnProperty vs in: in checks the chain, hasOwnProperty checks only the object
• Modifying Array.prototype or Object.prototype affects ALL instances — never do this in production
• __proto__ is deprecated — use Object.getPrototypeOf() and Object.setPrototypeOf()

Performance

V8 optimizes prototype lookups with hidden classes and inline caches. Long prototype chains (5+) can degrade performance. Keep inheritance shallow.