Question

I play javascript with the book Professional JavaScript for Web Developers. I practice an example in section 6.2.6. the codes are listed below：

function creatPrototype(subType, superType) 
{
    function subTypePrototype(){};
    subTypePrototype.prototype = superType.prototype;
    subTypePrototype.constructor = subType;
    subTypePrototype.str = "say";
    return new subTypePrototype();
}

function Person(name, age) 
{
    this.name = name;
    this.age = age;
}
Person.prototype.say = function(){
    writeln("bill say");
}


function itMan(name, age){
    Person.apply(this, arguments); 
}
itMan.prototype = creatPrototype(itMan, Person); 


var Bill = new itMan("bill", 25);

writeln(itMan.prototype.str);    //expect "say"
writeln(Person.prototype == itMan.prototype.prototype);   //expect true
Bill.say();  //expect "bill say"

the result is:

undefined

False

bill say

Why?

1. itMan.prototype.str is suppose to "say"

2. Person.prototype AND itMan.prototype.prototype should point to a same object

3. Bill.say() run correctly, so the prototype chain is OK.

Answer 1

You have to think about which property belongs to the constructor function and which one belongs to the instance. prototype is a property of the function, but constructor and str should be both properties of the instance.

This should do it:

function createPrototype(subType, superType) 
{
    function subTypePrototype(){};
    subTypePrototype.prototype = superType.prototype;

    var newPrototype = new subTypePrototype();

    newPrototype.constructor = subType;
    newPrototype.str = "say";
    return newPrototype;
}

But, as you are also passong subType, you can actually assign the prototype directly:

function inherit(subType, superType) 
{
    function tconstr(){};
    tconstr.prototype = superType.prototype;

    subType.prototype = new tconstr();

    subType.prototype.constructor = subType;
    subType.prototype.str = "say";
}

and then just call it with

inherits(itMan, Person);

---

Person.prototype AND itMan.prototype.prototype should point to a same object

Remember that prototype is a property of a function, not of objects. But itMan.prototype is a an object. You cannot access an objects prototype unless you explicitly refer to it (but I would not do so).

With ECMAScript 5, there is a way to get the prototype, using Object.getPrototypeOf ^[MDN]. This only works in newer browsers though.

Here is a working example of your code.