什么类型的对象是Javascript原型？

Question

function Person(name) {
  this.name = name;
}
var rob = new Person('Rob');

• Person is a function.
• Person is an object.
• Person is not a Person.
• rob is an object.
• rob's prototype ( __proto__ ) is Person.prototype, so rob is a Person.

But

console.log(Person.prototype);

outputs

Person {}

Is Person.prototype an object? Array? A Person?

If it is an Object, does that prototype also have a prototype?

Update on what I have learned from this question (Friday 24 January 2014, 11:38:26 AM)

function Person(name) {
  this.name = name;
}
var rob = new Person('Rob');

// Person.prototype references the object that will be the actual prototype (x.__proto__)
// for any object created using "x = new Person()". The same goes for Object. This is what
// Person and Object's prototype looks like.
console.log(Person.prototype); // Person {}
console.log(Object.prototype); // Object {}
console.log(rob.__proto__); // Person {}
console.log(rob.__proto__.__proto__); // Object {}

console.log(typeof rob); // object
console.log(rob instanceof Person); // true, because rob.__proto__ == Person.prototype
console.log(rob instanceof Object); // true, because rob.__proto__.__proto__ == Object.prototype

console.log(typeof rob.__proto__); // object
console.log(rob.__proto__ instanceof Person); // false
console.log(rob.__proto__ instanceof Object); // true, because rob.__proto__.__proto__ == Object.prototype

• Prototypes are just plain objects.
• typeof is useful to determine if something is object or primitive (and what sort of primitive it is), but useless to determine what sort of object it is.
• LHS instanceof RHS returns true if RHS.prototype turns up somewhere in the prototype chain of LHS.

Answer 1

Is Person.prototype an object?

Yes. Everything _{^{(interesting)}} is an object :-) See How is almost everything in Javascript an object? for details.

An Array?

No, definitively not.

A Person?

Depends. Most people would say it's not an instance of Person - it's the essence of all Persons (their prototype :-). However, console.log seems to identify it as such because it has a .constructor property pointing to the Person constructor.

If it is an Object, does that prototype also have a prototype?

Yes. Every Object does have a prototype. These build the so-called prototype chain , at whose end is a null reference. In your particular example, it's

      rob
       |
       v
 Person.prototype
       |
       v
 Object.prototype
       |
       v
      null

Answer 2

typeof Person === "function"
typeof Person.prototype === "object"