JavaScript Prototype and Inheritance: A Deep Dive

JavaScript uses prototype-based inheritance: objects don't have classes in the traditional sense; they have a hidden link to another object (the prototype), and property lookup walks that chain. Understanding prototypes is essential for debugging, extending built-ins, and choosing between class inheritance and composition. This post goes deep on the model and senior-level patterns.

How the prototype chain works

Every object has an internal slot [[Prototype]] (exposed in some environments as __proto__; the standard way to read it is Object.getPrototypeOf(obj)). When you access a property—e.g. obj.foo—the engine: