JavaScript Prototypes — What class Actually Does Under the Hood

"Explain prototypes" filters senior-track candidates. The mental model is one sentence: when a property isn't on an object, JavaScript looks up a chain of linked objects until it finds it or runs out.

Seeing the chain

const arr = [1, 2, 3];
arr.push(4);
// arr has no "push" — found on Array.prototype
// chain: arr → Array.prototype → Object.prototype → null

arr.toString();   // found two hops up, on Object.prototype... 
// (actually Array.prototype overrides it — closest wins!)

class is prototype sugar

class Student {
  constructor(name) { this.name = name; }
  greet() { return `Hi ${this.name}`; }
}
// is (almost) exactly:
function Student(name) { this.name = name; }
Student.prototype.greet = function () { return `Hi ${this.name}`; };

// greet lives ONCE on the prototype — 1000 students share it (memory win)

The naming confusion, resolved

• Student.prototype — the object instances will link to
• student.__proto__ (officially Object.getPrototypeOf) — the actual link on an instance
• extends chains prototypes: instance → Topper.prototype → Student.prototype → Object.prototype

Interview payoffs: explains why editing Array.prototype is dangerous, how polyfills work, and what hasOwnProperty actually checks.