Intanceof and the __proto__ and prototype

instanceof Operation code

1 functioninstance_of (L, R) {2 //L = left expression, R for right expression3  varO =R.prototype;4 //the display prototype of R is taken5L =l.__proto__;6 //an implicit prototype of L7   while(true) { 8    if(L = = =NULL) 9      return false; Ten    if(O = = =L) One //Here's the point: Returns True when O is strictly equal to L A      return true;  -L =l.__proto__; -  }  the}

var function (name) {    this. Name = name;}  var New Person (); The operation of the new operator is var p ==  Person.prototypePerson.call (p)

var p={}; In other words, initialize an object p.

p.__proto__ = Person.prototype;

Person.call (P); that is, construct p, which can also be called initialization p.

The key is the second step, and let's prove it:

var function () {}varnew=  person.prototype)

This piece of code returns TRUE. Explain the correctness of step 2.

So what is __proto__? Let's simply say here. Each object initializes a property inside it, that is, __proto__, when we access the property of an object, if the object does not exist inside this property, then he will go to __proto__ to find this attribute, this __proto__ will have their own __proto__, So we have been looking for the concept of the prototype chain we usually call.

According to the standard, __proto__ is not public, that is, a private property, but the engine of Firefox exposes him to become a common attribute, we can access and set up externally.

Well, the concept is clear, let's take a look at the following code:

var function  function  () {    alert ("person say");} var New

This code is very simple, I believe everyone has written this, so let's see why p can access the person's say.

First, var p=new person (); P.__proto__=person.prototype can be obtained. So when we call P. Say (), first there is no Say this attribute in P, so, he needs to go to his __proto__ to find, that is, person.prototype, and we define the above person.prototype.say=function () {}; So, we found this method.

OK, next, let's look at a more complicated one.

varperson =function () { }; Person.prototype.Say=function() {alert ("Person say");} Person.prototype.Salary= 50000;varProgrammer =function () { }; Programmer.prototype=NewPerson (); Programmer.prototype.WriteCode=function() {alert ("Programmer writes Code");}; Programmer.prototype.Salary= 500;varp =NewProgrammer ();p. Say ();p. Writecode (); alert (p.salary); 

Let's do this derivation:

var p=new Programmer () can be obtained p.__proto__=programmer.prototype;

And on top we specify programmer.prototype=new person (); Let's split this up, Var p1=new person (); PROGRAMMER.PROTOTYPE=P1; then:

P1.__proto__=person.prototype;

Programmer.prototype.__proto__=person.prototype;

Get P.__proto__=programmer.prototype on top of the above. Can get p.__proto__.__proto__=person.prototype.

Well, after figuring it out, let's look at the results above, P.say (). Because P does not say this attribute, so go to p.__proto__, that is, Programmer.prototype, that is, p1 to find, because there is no P1 say, then go to p.__proto__.__proto__, That is person.prototype to find, so found the alert ("Person say") method.

The rest is the same.

Transfer from http://www.cnblogs.com/zzcflying/archive/2012/07/20/2601112.html

Intanceof and the __proto__ and prototype