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From: JJ <jaejunks@nah.meh>
Newsgroups: comp.lang.javascript
Subject: Re: Does copying the prototype reference from another constructor function maintain the prototype chain?
Date: Sat, 5 Jan 2013 10:57:02 +0000 (UTC)
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GGolf <invalid@invalid.com> wrote:
> I've been a Javascript dabbler for a few years, and have just embarked 
> on learning more advanced stuff.
> 
> I have the following question in a tutorial by John Resig:
> 
> At http://ejohn.org/apps/learn/#76, Resig mentions that only the 
> following maintains the prototype chain:
> 
> Ninja.prototype = new Person();
> 
> However, I notice that even the following statement makes all the test 
> statements pass:
> 
> Ninja.prototype = Person.prototype;
> 
> Does it mean that using "Person.prototype" can actually be used instead 
> of "new Person()"?

Yes.

"Ninja.prototype = Person.prototype;" does create prototype inheritance, 
but with only 2 levels: Person and Object. The inheritance from Ninja and 
Person is the exact same.

function Person(){}

function Ninja(){}
Ninja.prototype=new Person();

var ninjaNew=new Ninja();
console.log('ninjaNew');
console.log(
  ninjaNew.__proto__===Ninja.prototype
); //true
console.log(
  ninjaNew.__proto__.__proto__===Person.prototype
); //true
console.log(
  ninjaNew.__proto__.__proto__.__proto__===Object.prototype
); //true

Ninja.prototype=Person.prototype;
var ninjaProto=new Ninja();
console.log('ninjaProto');
console.log(
  ninjaProto.__proto__===Ninja.prototype
); //true
console.log(
  ninjaProto.__proto__===Person.prototype
); //true
console.log(
  ninjaProto.__proto__.__proto__===Person.prototype
); //false
console.log(
  ninjaProto.__proto__.__proto__===Object.prototype
); //true