Classes as Prototype Sugar

## What the declaration actually creates A class declaration creates exactly one function object. `class Point { constructor(x) { this.x = x } move(dx) { this.x += dx } }` produces a function named `Point` whose body is the `constructor` body; every other method in the class body becomes a property of `Point.prototype`; `Point.prototype.constructor` refers back to `Point`; and `new Point(1)` produces an object whose internal prototype is `Point.prototype`, so `p.move(2)` is resolved by walking the prototype chain rather than found on the instance. `typeof Point` is `"function"`. That is the same object graph the pre-class pattern builds by hand: ```js function Point(x) { this.x = x; } Point.prototype.move = function (dx) { this.x += dx; }; ``` So the shape is identical. What differs is *how* the method property is installed, and a handful of restrictions the class form adds. ## The descriptor difference A method in a class body is defined, not assigned — the effect is the same as `Object.defineProperty(Point.prototype, 'move', { value: fn, writable: true, enumerable: false, configurable: true })`. A hand-written `Point.prototype.move = fn` is an ordinary assignment, and assignment creates a data property with `writable`, `enumerable` and `configurable` all `true`. The single practical bit is `enumerable`: ```js Object.keys(Point.prototype); // [] Object.getOwnPropertyNames(Point.prototype); // ['constructor', 'move'] ``` ## Why non-enumerability matters `for...in` iterates enumerable string-keyed properties *including inherited ones*. With the ES5 pattern, `for (const k in p)` yields `'move'` alongside the instance's own data — which is exactly why old code is littered with `if (!obj.hasOwnProperty(k)) continue;`. With a class, the loop yields only the instance's own properties, and the guard is unnecessary. The same rule drives copying helpers. `Object.assign` and object spread copy *own enumerable* properties, so `Object.assign({}, Legacy.prototype)` carries the methods over, while `Object.assign({}, Point.prototype)` copies nothing at all. Anyone who wrote a hand-rolled "mix these methods in" helper on top of `Object.assign` finds it silently produces empty objects the day the source becomes a class. The fix is to read keys with `Object.getOwnPropertyNames` (or `Reflect.ownKeys`, which also returns symbols) and copy descriptors with `Object.getOwnPropertyDescriptor` / `Object.defineProperty`. The class default is not arbitrary: the built-ins behave the same way. `Array.prototype.map` and `Object.prototype.toString` are non-enumerable, which is why `for...in` over an array does not list `map`. Class syntax simply made user code match the platform. ## Methods are not constructors A method defined in a class body (like a shorthand method in an object literal) has no `[[Construct]]` internal method: ```js const p = new Point(1); new p.move(); // TypeError: p.move is not a constructor ``` The assigned function expression in the ES5 version *is* constructible, and it additionally allocates its own `.prototype` object that nothing ever uses. Class methods also have no `prototype` property at all. ## The prototype property itself For an ordinary function, `Foo.prototype` is `{ writable: true, enumerable: false, configurable: false }` — the classic `Foo.prototype = { ... }` wholesale replacement works (and quietly loses `constructor` unless you restore it). For a class, `prototype` is `{ writable: false, enumerable: false, configurable: false }`, so `Point.prototype = {}` throws a `TypeError` in strict code and fails silently in sloppy code. You can still patch individual members — class methods are writable and configurable, so `Point.prototype.move = betterMove` and `delete Point.prototype.move` both work. ## What is genuinely the same Method lookup through the chain, `instanceof`, the single shared copy of each method, and the ability to monkey-patch the prototype after the fact are all unchanged. The mental model "a class is a constructor function plus a prototype object" stays correct; what you must add is "…whose methods are defined rather than assigned, and whose function object refuses a few things an ordinary function allows."