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What is the difference between `__proto__`, `.prototype`, and `Object.getPrototypeOf()`?
One lives on instances, one on constructor functions, one is the standard API.
Ideal Answer
These three names show up together in interviews but refer to different things:
| What it is | Where it lives | |
|---|---|---|
Object.getPrototypeOf(obj) |
Standard way to read an instance's [[Prototype]] |
Any object |
__proto__ |
Legacy accessor to the same [[Prototype]] link |
Any object (avoid in new code) |
Constructor.prototype |
The object used as [[Prototype]] for instances created with new Constructor() |
Function objects only |
JavaScript
function Dog() {}
const rex = new Dog()
Object.getPrototypeOf(rex) === Dog.prototype // true
rex.__proto__ === Dog.prototype // true (legacy)
Dog.prototype.bark = () => 'woof'
rex.bark() // 'woof'Common mistake: Dog.prototype is not the prototype of Dog itself — it is the prototype object assigned to instances. The prototype of the function Dog is Function.prototype.
Prefer Object.getPrototypeOf() over __proto__ in production code.